Abstract Predictions of streamwater acidification using the MAGIC model, for the British upland regions sensitive to acidic deposition are presented. They provide examples of how both acidic deposition and conifer afforestation can increase streamwater acidity. Long term trends are predicted. Model simulations demonstrate the relative effects of sea salt and acidic oxide inputs. The study highlights the need for regional surveys of chemical and hydrological catchment characteristics and the need for applications of the models at a regional scale.