diff --git a/dm-serializer.gemspec b/dm-serializer.gemspec
index dd058db..7c23fd2 100644
--- a/dm-serializer.gemspec
+++ b/dm-serializer.gemspec
@@ -15,10 +15,17 @@ Gem::Specification.new do |gem|
   gem.require_paths = [ "lib" ]
   gem.version       = '1.2.2'
 
-  gem.add_runtime_dependency('fastercsv',  '~> 1.5')
+  # Ruby 3.3 compatibility: the original 1.2.2 gemspec pinned json (~> 1.6),
+  # json_pure (~> 1.6) and fastercsv (~> 1.5). All three are unmaintained and
+  # incompatible with Ruby 3.x:
+  #   * json/json_pure 1.8.x use a Parser whose #parse arity differs from modern
+  #     JSON, and json_pure 1.8.6 fails to load on Ruby 3.2+ (3-arg Regexp.new etc.).
+  #   * fastercsv targets Ruby 1.8 only; Ruby 1.9+ ships csv in the stdlib.
+  # The runtime never uses these gems directly: to_json.rb uses multi_json, and
+  # to_csv.rb does `require 'csv'` (the stdlib) on Ruby >= 1.9. We drop the
+  # json/json_pure/fastercsv pins and rely on multi_json + the stdlib csv, which
+  # lets the bundle resolve the modern default json gem instead of json 1.8.6.
   gem.add_runtime_dependency('multi_json', '~> 1.0')
-  gem.add_runtime_dependency('json',       '~> 1.6')
-  gem.add_runtime_dependency('json_pure',  '~> 1.6')
   gem.add_runtime_dependency('dm-core',    '~> 1.2.0')
 
   gem.add_development_dependency('rake',  '~> 0.9')