The Ontario acid sensitivity database (OASD) is a compilation of chemical data collected on about 6000 lakes, derived from a number of studies, most of them conducted as part of the Acid Precipitation in Ontario study (APIOS). This report describes the water chemistry of those lakes, giving summaries of the data and studies used. Details are given on the measured lake chemistry for cations, anions, metals, conductivity, apparent and true colour and dissolved organic carbon. Sulphur deposition zones are detailed, along with the chemical characteristics of lakes by such zones. The report concludes with estimates of lake resources affected by acid deposition.