diff --git a/tests/test_conversion.py b/tests/test_conversion.py
new file mode 100644
index 0000000..0095f07
--- /dev/null
+++ b/tests/test_conversion.py
@@ -0,0 +1,157 @@
+"""Tests for conversion service module."""
+
+import pytest
+from unittest.mock import MagicMock, patch
+
+from codexchange.models import Language, ConversionRequest, ConversionResult
+from codexchange.services.conversion_service import ConversionService
+
+
+class TestConversionService:
+    """Tests for ConversionService."""
+
+    def test_build_conversion_prompt(
+        self,
+        mock_ollama_service,
+        sample_python_code
+    ):
+        """Test building conversion prompt."""
+        service = ConversionService(ollama_service=mock_ollama_service)
+        
+        prompt = service.build_conversion_prompt(
+            sample_python_code,
+            Language.PYTHON,
+            Language.JAVASCRIPT
+        )
+        
+        assert "python" in prompt
+        assert "javascript" in prompt
+        assert "IMPORTANT REQUIREMENTS" in prompt
+        assert "Preserve ALL comments" in prompt
+        assert sample_python_code in prompt
+
+    def test_convert_success(
+        self,
+        mock_ollama_service,
+        conversion_request
+    ):
+        """Test successful conversion."""
+        mock_ollama_service.generate.return_value = "converted code"
+        
+        service = ConversionService(ollama_service=mock_ollama_service)
+        result = service.convert(conversion_request)
+        
+        assert result.success is True
+        assert result.converted_code == "converted code"
+        assert result.error_message is None
+
+    def test_convert_failure(
+        self,
+        mock_ollama_service,
+        conversion_request
+    ):
+        """Test failed conversion."""
+        mock_ollama_service.generate.side_effect = Exception("Connection failed")
+        
+        service = ConversionService(ollama_service=mock_ollama_service)
+        result = service.convert(conversion_request)
+        
+        assert result.success is False
+        assert result.error_message is not None
+
+    def test_convert_with_retry(
+        self,
+        mock_ollama_service,
+        conversion_request
+    ):
+        """Test conversion with retry on failure."""
+        mock_ollama_service.generate.side_effect = [
+            Exception("fail 1"),
+            Exception("fail 2"),
+            "success on third try"
+        ]
+        
+        service = ConversionService(ollama_service=mock_ollama_service)
+        result = service.convert(conversion_request)
+        
+        assert result.success is True
+        assert result.converted_code == "success on third try"
+
+    def test_clean_conversion_output_with_code_blocks(self):
+        """Test cleaning conversion output with code blocks."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        output = """Here is the converted code:
+ ```javascript
+ console.log("Hello");
+ ```
+"""
+        cleaned = service._clean_conversion_output(output)
+        assert cleaned == 'console.log("Hello");'
+
+    def test_clean_conversion_output_without_code_blocks(self):
+        """Test cleaning conversion output without code blocks."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        output = "console.log('Hello');"
+        cleaned = service._clean_conversion_output(output)
+        assert cleaned == "console.log('Hello');"
+
+    def test_count_comments_python(self):
+        """Test counting Python comments."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        code = "# This is a comment\nprint('hello')"
+        count = service.count_comments(code)
+        assert count == 1
+
+    def test_count_comments_javascript(self):
+        """Test counting JavaScript comments."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        code = """
+ // This is a comment
+ function hello() {
+     /* Block comment */
+     return 1; // inline comment
+ }
+"""
+        count = service.count_comments(code)
+        assert count == 3
+
+    def test_verify_comment_preservation(self):
+        """Test comment preservation verification."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        original = "# Comment\nprint('hello')"
+        converted = "// Comment\nconsole.log('hello')"
+        
+        result = service.verify_comment_preservation(original, converted)
+        assert result is True
+
+    def test_verify_comment_preservation_no_comments(self):
+        """Test comment preservation with no comments."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        original = "print('hello')"
+        converted = "console.log('hello')"
+        
+        result = service.verify_comment_preservation(original, converted)
+        assert result is True
+
+    def test_verify_comment_preservation_missing_comments(self):
+        """Test comment preservation when comments are missing."""
+        mock_service = MagicMock()
+        service = ConversionService(ollama_service=mock_service)
+        
+        original = "# Comment\nprint('hello')"
+        converted = "console.log('hello')"
+        
+        result = service.verify_comment_preservation(original, converted)
+        assert result is False