The role of organic and minerogenic acidity in controlling pH levels in Finnish rivers was studied by measuring total organic carbon (TOC) and non-marine sulphate (*SO(4)) concentrations in river water of the main river basins and their sub-basins. The basins are located along a latitudinal gradient (60 degrees N to 69 degrees N) and cover 297,322 km(2), 88% of the total area of Finland. The basins are predominately covered by coniferous forests and peatlands, and are located in areas with low sulphate deposition (80-430 mg S m(-2) a(-1)). The proportion of the basin covered by forests on mineral soils ranges from 29 to 64% and the proportion of peatlands from 3 to 60%. The percentage of peatland is highest in northern Finland (28-60%), whereas the proportion of forests on mineral soils increases towards the south. The majority of the agricultural land is located in southern and western Finland. *SO(4) concentrations were positively correlated with the proportion of agricultural land in the basin. Moreover, the percentage of peatlands had a positive correlation with the concentrations of TOC and organic anion. High peatland proportion and high TOC and organic anion concentrations decreased pH values in the river water, whereas no correlation between *SO(4) concentrations and pH was observed. The average organic anion concentration exceeded the average *SO(4) concentration in river water in 17 basins out of the 86 studied basins. The organic anion-dominated areas were situated in northern basins, reflecting the high extent of peatlands in these areas. *SO(4) dominated in southern Finland and in western coastal areas, where the extent of agricultural land, acid sulphate soils and the deposition of sulphate is highest.