The categorization of documents into subject-specific categories is a useful enhancement for large document collections addressed by information retrieval systems, as a user can first browse a category tree in search of the category that best matches her interests, and then issue a query for more specific documents ``from within the category''. This approach combines two modalities in information seeking that are most popular in Web-based search engines, i.e. category-based site browsing (as exemplified by e.g. {sc Yahoo}$^{smallsc TM}$) and keyword-based document querying (as exemplified by e.g. {sc AltaVista}$^{smallsc TM}$). Appropriate query expansion tools need to be provided, though, in order to allow the user to incrementally refine her query through further retrieval passes, thus allowing the system to produce a series of subsequent document rankings that hopefully converge to the user's expected ranking. In this work we propose that automatically generated, category-specific ``associative'' thesauri be used for such purpose. We discuss a method for their generation, and discuss how the thesaurus specific to a given category may usefully be endowed with "gateways" to the thesauri specific to its parent and children categories