Simple hydrochemical reservoir models based on the mobile anion concept are described for the Birkenes and Storgama catchments in southern Norway with acidified streamwater. Key processes modelled include water routing, sulphate adsorption-desorption, ion-exchange reactions, weathering and a gibbsite equilibrium condition. The models reproduce much of the daily and seasonal variations in streamwater chemistry which have been observed over several years. Model considerations have been used in an attempt to explain the acidification of streamwater in these two areas. It is hypothesized that soil acidification has occurred and that acid precipitation is at least partly responsible. The results are tentative largely because the models are derived from present-day streamwater chemistry.