MAGIC, a hydrochemical model of catchment acidification has been linked to a model, PROFILE, which estimates catchment weathering rates, in an application to a small forested catchment in the Montseny mountains of Catalonia (northeast Spain), to assess the effects of climate change on streamwater chemistry for a montane-Mediterranean region. Two scenarios of climate change are considered: a 4°C temperature increase combined with 10% precipitation increase and a 4°C temperature increase combined with 10% precipitation decrease. The modelled effect of a 10% precipitation increase on the water partition between runoff and evapotranspiration is a slight dilution of the streamwater chemistry. Correspondingly, the modelled effect of a 10% precipitation decrease is a sharp concentration increase in streamwater chemistry. Weathering rates increase with increased temperature and increased precipitation but are almost unaffected by increased temperature and decreased precipitation. Therefore, changes in streamwater concentrations, linked to the effect of weathering changes, appear only in the scenario with a warmer and wetter climate. The approach has also been used to assess the effects of changing air mass patterns caused by climate change as an increase in alkaline-rich rains associated with Sahelo-Saharan dust may well result. This study indicates that the main chemical changes following climate change will be associated with weathering rate changes rather than with changes in the atmospheric supply of base cations and alkalinity. The case for considering the effects of temperature and moisture availability in climate change models of soil and streamwater quality is shown to be overwhelming as rates of weathering are strongly influenced by these factors. © 1996 - Elsevier Science B.V. All rights reserved.