A unique procedure for determining higher carboxylic acids and carboxyl groups in motor and food oils, gasolines, and organic reagents is proposed, which is based on the reaction in toluene-isopropanol solution of acid groups with the ion associate of bromothymol blue with trinonyloctadecylammonuim as a colored reagent. The method is characterized by high sensitivity, ease of operation, and suitability for proximate routine analysis.