Abstract: In order to determine the carboxyl groups content in low- rank coals accurately, ion exchange methods was taken to determine the carboxyl groups in Ba ori brown coal and Pingshuo long flame coal. The effect of ion exchange time, wetting agent ( methanol) and pre- acid- extraction was researched. It was concluded tha t the ion exchange could get completed when ion exchange time was prolonged. The quantity of carboxyl groups detected stay constant when ion exchange time was m ore than 15 hours. The effect of wetting agent was obvious when ion exchange time was short. While, as the ion exchange time was prolonged, its effect got less obviou s. The error caused by inorganic substance could be eliminated by pre- extracting with acid, which made the results more accurate.