A novel semantic caching scheme suitable for web database environments is proposed. In our scheme, tasks for query translation/capability mapping (named as "query naturalization") between wrappers and web sources and tasks for semantic caching are seamlessly integrated, providing greater levels of query optimization opportunities. Semantic cache consists of three components: (1) "semantic view ", a description of the contents in the cache using sub-expressions of the previous queries, (2) "semantic storage", a placeholder for data satisfying the semantic view, and (3) "physical storage", a storage containing tuples (or objects) that are shared by all semantic storages in the cache. Because of the inherent characteristics of IR systems, issues similar to the classical query containment problem arise. Possible match types and detailed algorithms for comparing the input query with stored semantic views are discussed. We consider these issues in the context of a prototype web dat..