Solutions of labelled benzyl mercaptan-(35S) in water and cream were subjected to steam distillation in a continuous vaporization equilibrium still, and the concentrations of mercaptan in distillates and residues were obtained by measurement of the radioactivity in toluene extracts.

In the steam distillation from aqueous solution the vapour/liquid equilibrium relationship was not linear. The vapour/liquid equilibrium coefficient increased from 73 to 108 over the range of concentrations 0·01–0·05 ppm. in the liquid fraction at the time of partition.

In the steam distillation from cream containing 30% fat the relationship was linear, with an equilibrium coefficient of 1·35 over the range 0·1–0·6 ppm. in the cream at the time of partition.

The values of the vapour /liquid equilibrium coefficients are of the same order as those reported previously, 105 and 1·30, respectively, for which estimations were made by an absorption method.

The butterfat/water partition coefficient for benzyl mercaptan was found to be 160, which is consistent with the large difference found between the steam volatilities of the substance from water and from cream.