Quantitative predictions of the effects of acid deposition on terrestrial and aquatic systems require physically based, process-orientated models of catchment soil water and streamwater chemistry. A desirable characteristic of such models is that they include terms to describe the important phenomena controlling a system's chemical response to acidic deposition, yet be restricted in complexity so that they can be implemented on diverse systems with a minimum of a prior data. Presented is an assessment of a conceptual model of soil water and streamwater chemistry based on soil cation exchange, dissolution on aluminum hydroxide, and solution of carbon dioxide, all processes that occur in catchment soils and that have rapid equilibration times. The model is constructed using an 'average' or lumped representation of these spatially distributed catchment processes.