The eastern half of the North American continent experiences acidic rain. An approach to predicting that acidity from SO 2 emission rates and meteorological data is suggested. It is based on modelling the atmospheric sulphur cycle and on the strong correlation between hydrogen and sulphate ions in rain collected by the CANSAP and MAP3S precipitation networks. The molar ratio [H +] [SO 4 2−] in rain is commonly 1.6–2 in the northeastern U.S.A. and ~1 in acid sensitive areas of eastern Canada. Using the solution chemistry of sulphur dioxide, it is concluded that dissolved SO 2 can contribute substantially to the acidification of a receptor by rain especially at low temperatures in areas having particulate matter with a low hydrogen ion content. An expression is derived relating SO 2 washout ratio to two parameters, rain water pH and temperature. Ways of incorporating SO 2 washout into sulphur cycle models are discussed.