We propose a simple yet eective approach to context sensitive synonym discovery for Web search queries based on co-click analysis; i.e., analyzing queries leading to clicking same documents. In addition to deriving word based synonyms, we also derive concept based synonyms with the help of query segmentation. Evaluation results show that this approach dramatically outperforms the thesaurus based synonym replacement method in keeping search intent, from accuracy of 40% to above 80%. Categories and Subject Descriptors: H.3.3 [Information Storage and Retrieval]: Information Search and Retrieval General Terms: Algorithms, Experimentation