We present a deductive data model for concept-based query expansion. It is based abstraction levels: the conceptual, the expression and the occurrence level. Concepts and their relationships are represented at the conceptual level. The expression level represents natural language expressions for concepts. Each expression has one or more matching models at the occurrence level. Each model specifies the matching of the expression in database indices built in varying ways. The data model supports a concept-based query expantion and formulation tool, the ExpansionTool, for environments providing heterogeneous IR systems. Expansion is controlled by adjustable matching reliability. Published in: H.-;P. Frei, D. Harman, P. Sch