The method of determining mandelic acid in urine after Engstöm was examined. It was stated that supplementing the procedure by introducing an internal standard (a-naphthol) improves the accurary of the method. In case of a full procedure for mandelic acid solutions in urine we get the range of 80 to 800 mg/l +/- 7,13%. This method detectability is sufficient for an industrial evaluation of the exposure to styrene or ethylbenzene.