Presents a user-centered investigation of interactive query expansion within the context of a relevance feedback system. Data were collected from 25 searches using the INSPEC database. Results discuss issues relating to query expansion, retrieval effectiveness, correspondence of the online-to-offline relevance judgments, and selection of terms for query expansion by users (interactive query expansion). Results provide evidence for the effectiveness of interactive query expansion. (Contains 49 references.) (Author/AEF)