Module-oriented modeling of reactive transport with HYTEC. The paper introduces HYTEC, a coupled reactive transport code currently used for groundwater pollution studies, safety assessment of nuclear waste disposals, geochemical studies and interpretation of laboratory column experiments. Based on a known permeability field, HYTEC evaluates the groundwater flow paths, and simulates the migration of mobile matter (ions, organics, colloids) subject to geochemical reactions. The code forms part of a module-oriented structure which facilitates maintenance and improves coding flexibility. In particular, using the geochemical module CHESS as a common denominator for several reactive transport models significantly facilitates the development of new geochemical features which become automatically available to all models. A first example shows how the model can be used to assess migration of uranium from a sub-surface source under the effect of an oxidation front. The model also accounts for alteration of hydrodynamic parameters (local porosity, permeability) due to precipitation and dissolution of mineral phases, which potentially modifies the migration properties in general. The second example illustrates this feature.