Dimethylformamide dialkylacetals have been found to react readily with primary sulfonamides to form N-dimethylaminomethylene derivatives. These compounds possess excellent gas-liquid chromatographic properties and can be conveniently prepared at the submicrogram level. Their retention times are much greater than those of other sulfonamide derivatives (e.g., N,N-dimethyl) but their ease of preparation and lack of absorptive properties make them attractive for gas-liquid chromatographic studies. The practical applicability of this derivatization approach to biological studies is illustrated by the gas-liquid chromatographic determination of 3-bromo-5-cyanobenzenesulfonamide in ovine blood using 3,5-dibromobenzenesulfonamide as the internal standard. The method has a detection limit of 25 ppb with electron capture detection.