Activity coefficients of the binary liquid mixtures benzene-toluene and acetonechloroform were determined using thermal desorption modulation for direct headspace sampling into a capillary gas chromatograph. A thermal desorption modulator is a short, heated section at the head of the column. An electrical current pulse applied to a thin conductive film heats the modulator section and the stationary phase within it, releasing any retained substances as a concentration pulse which flows into the column. The modulator acts like an automatic and highly reproducible small-volume injector for a continuously flowing sample stream. Short-term relative standard deviations obtained using this technique are approximately 2%.