Samples on acid content of particulates and rainfall are collected and analysed at nine stations in Bangkok Metropolitan area. The average concentrations of particulate acidity vary from 5.38 to 10.15 μg/m3and are comparable with those reported for other cities in the world. It is found that weighted average concentrations of acidity of rain water in terms of pH values at the sampling stations vary from 5.57 to 6.32. Chemical analyses of individual rainfalls also show a close correlation between pH and sulphate content (SO4—) of rain water. A linear regression between pH values and acid contents of particulate yields the following relationship: