A specific and practicable method is described for the quantitative determination of urinary phenol- and indole-carboxylic acids. High specificity is achieved by a preliminary separation of the free acids (extracted from the urine sample) with the aid of organophilic gel chromatography on Sephadex LH 20, followed by gas chromatographic analysis of the silyl derivatives of the acids. The organophilic gel chromatography of the free acids shows a high recovery rate in the micro- and submicrogram range. The difficulties encountered in other techniques in the derivatisation and gas chromatographic separation of the individual components are avoided by using the preliminary separation, and by using N-methyl-N-trimethylsilyl-trifluoroacetamide for the derivatisation. Use of this preparation technique with a direct read-out gas chromatograph with automatic sample introduction, gives high accuracy and precision, and a facility for the determination of a wide range of aromatic acids in urine.