As a prelude to the study of chemical weathering of rocks and soils in S.E. Ontario, a study of the composition of precipitation waters at 12 sites within the area was initiated. Analyses of bulk precipitation of both rainfall and snowfall are presented and compared with similarly derived results in other areas.The silica and aluminum contents are regarded as being high, although in snow silica is absent and aluminum is quite high. Areal differences in silica content appear to be connected with local differences in lithology. Iron occurs in the same concentrations as aluminum and is probably derived from atmospheric dusts. The sodium content is high, especially when compared with inland sites in other areas. Potassium is generally somewhat lower than sodium. Although the calcium content may reflect local differences in lithology, the dissolved magnesium is surprisingly uniform. Lithological differences in the area are also reflected in the significantly higher nitrate-nitrogen concentrations in the limestone area. Snowfall is markedly high in sulfates, which is considered to be a function of climatic and industrial causes. The high chloride concentrations in such a 'non-marine' environment are not explained.