The exploration of large information spaces is a difficult task, especially if the user is not familiar with the terminology used to describe information. Conceptual models of a domain in terms of thesauri or ontologies can leverage this problem to some extend. In order to be useful, there is a need for interactive tools for exploring large information sets based on conceptual knowledge. We present a thesaurus based browser that supports a mixed-initiative exploration of large online resources that provides support for thesaurus-based search and topic-based exploration of query results. We motivate the chosen exploration strategy the browser functionality, present the results of user studies and discuss future improvements of the browser.