A study of the spatio-temporal variation in hydrochemistry in the afforested catchment of the River Douglas, in the Araglin Valley, Co. Cork, Ireland, was undertaken over a two year period. The aim of the study was to examine the influence of afforestation on stream water quality both spatially and temporally. The catchment, one of the most westerly in Europe, with low atmospheric pollution, allowed the analysis of the interactions between conifer afforestation per se on stream chemistry. In contrast to most other studies, there was a general trend of increasing pH and related variables with distance from headwater despite increasing levels of catchment afforestation. In one tributary, pH and related variables increased rapidly as the stream entered the forest, with pH rising by 1.67 units over a distance of 1.2 km. Temporal fluctuations in most physico-chemical variables were minor and no acid pulses were noted during spate. Thus, the current level of afforestation within the River Douglas catchment does not appear to have negatively affected stream chemistry. © 1998 Kluwer Academic Publishers.