diff --git a/src/srf.zig b/src/srf.zig
index 5e9ad75..f116a3e 100644
--- a/src/srf.zig
+++ b/src/srf.zig
@@ -346,7 +346,19 @@ pub const Record = struct {
         return null;
     }
 
+    fn maxFields(comptime T: type) usize {
+        const ti = @typeInfo(T);
+        if (ti != .@"union") return std.meta.fields(T).len;
+        comptime var max_fields = 0;
+        inline for (std.meta.fields(T)) |f| {
+            const field_count = std.meta.fields(f.type).len;
+            if (field_count > max_fields) max_fields = field_count;
+        }
+        return max_fields + 1;
+    }
+
     fn OwnedRecord(comptime T: type) type {
+        // for unions, we don't know how many fields we're dealing with...
         return struct {
             fields_buf: [fields_len]Field,
             fields_allocated: [fields_len]bool = .{false} ** fields_len,
@@ -355,7 +367,7 @@ pub const Record = struct {
             cached_record: ?Record = null,
 
             const Self = @This();
-            const fields_len = std.meta.fields(T).len;
+            const fields_len = maxFields(T);
 
             pub const SourceType = T;
 
@@ -378,8 +390,8 @@ pub const Record = struct {
                 comptime field_name: []const u8,
                 comptime field_type: type,
                 comptime default_value_ptr: ?*const anyopaque,
+                val: field_type,
             ) !usize {
-                const val = @field(self.source_value, field_name);
                 if (default_value_ptr) |d| {
                     const default_val: *const field_type = @ptrCast(@alignCast(d));
                     if (std.meta.eql(val, default_val.*)) return inx;
@@ -434,21 +446,46 @@ pub const Record = struct {
                 }
             }
             pub fn record(self: *Self) !Record {
+                return self.recordInternal(SourceType, self.source_value, 0);
+            }
+            fn recordInternal(self: *Self, comptime U: type, val: U, start_inx: usize) !Record {
                 if (self.cached_record) |r| return r;
-                var inx: usize = 0;
-                const ti = @typeInfo(SourceType);
+                var inx: usize = start_inx;
+                const ti = @typeInfo(U);
                 switch (ti) {
                     .@"struct" => |info| {
-                        inline for (info.fields) |f|
-                            inx = try self.setField(inx, f.name, f.type, f.default_value_ptr);
+                        inline for (info.fields) |f| {
+                            const field_val = @field(val, f.name);
+                            inx = try self.setField(inx, f.name, f.type, f.default_value_ptr, field_val);
+                        }
                     },
                     .@"union" => |info| {
-                        inline for (info.fields) |f|
-                            inx = try self.setField(inx, f.name, f.type, null);
+                        const active_tag_name = @tagName(val);
+                        comptime var has_decl = false;
+                        inline for (info.decls) |d| {
+                            if (comptime std.mem.eql(u8, "srf_tag_field", d.name)) has_decl = true;
+                        }
+                        const key = if (has_decl)
+                            U.srf_tag_field
+                        else
+                            "active_tag";
+                        self.fields_buf[inx] = .{
+                            .key = key,
+                            .value = .{ .string = active_tag_name },
+                        };
+                        inx += 1;
+                        switch (val) {
+                            inline else => |payload| {
+                                if (@typeInfo(@TypeOf(payload)) == .@"union")
+                                    @compileError("Nested unions not supported for srf serialization");
+                                return self.recordInternal(@TypeOf(payload), payload, inx);
+                            },
+                        }
                     },
                     .@"enum" => |info| {
+                        // TODO: I do not believe this is correct
                         inline for (info.fields) |f|
-                            inx = try self.setField(inx, f.name, self.SourceType, null);
+                            inx = try self.setField(inx, f.name, self.SourceType, null, val);
                     },
                     .error_set => return error.ErrorSetNotSupported,
                     else => @compileError("Expected struct, union, error set or enum type, found '" ++ @typeName(T) ++ "'"),
@@ -1246,6 +1283,41 @@ test "serialize/deserialize" {
     ;
     try std.testing.expectEqualStrings(expect, compact_from);
 }
+test "unions" {
+    const Foo = struct {
+        number: u8,
+        true_or_false: bool,
+    };
+    const Bar = struct {
+        sentence: []const u8,
+        decimal: f64,
+    };
+    const MixedData = union(enum) {
+        foo: Foo,
+        bar: Bar,
+
+        //pub const srf_tag_field = "foobar";
+    };
+
+    const data: []const MixedData = &.{
+        .{ .foo = .{ .number = 42, .true_or_false = true } },
+        .{ .bar = .{ .sentence = "foobar", .decimal = 6.9 } },
+    };
+    const alloc = std.testing.allocator;
+    var buf: [4096]u8 = undefined;
+    const compact_from = try std.fmt.bufPrint(
+        &buf,
+        "{f}",
+        .{fmtFrom(MixedData, alloc, data, .{})},
+    );
+    const expect =
+        \\#!srfv1
+        \\active_tag::foo,number:num:42,true_or_false:bool:true
+        \\active_tag::bar,sentence::foobar,decimal:num:6.9
+        \\
+    ;
+    try std.testing.expectEqualStrings(expect, compact_from);
+}
 test "compact format length-prefixed string as last field" {
     // When a length-prefixed value is the last field on the line,
     // rest_of_data.len == size exactly. The check on line 216 uses