A method is described for the quantitative assay of the methylation of alkanethiols from the methyl-donor S-adenosylmethionine, catalysed by the microsomal enzyme Sadenosyl-L-methionine:thiol S-methyltransferase (E.C. 2. I. 1.9). The reaction is carried out in sealed vials, one fifth pf whose volume is taken up by an aqueous phase containing the enzyme and reactants. The volatile substrates and products of the reaction, thiols and thioethers, respectively, are present in equilibrium both in the liquid and gas phases in the reaction vessels. Aliquots of the gas phase are removed at intervals in gas-tight syringes, and analysis is performed directly on a gas chromatograph fitted with a flame-ionization detector. The amounts of thiol and thioether detected are then related to the total amounts of substance in the reaction vessels from calibration measurements, so that the kinetics of the enzymatic process can be evaluated. This technique oEe:s distinct advantages over previously reported methods, in that no radioactively labelled compounds are required. Furthermore, decreases in substrate and increases in pro'duct can be assayed simultaneously, and the methylation of a mixture of thiols can be monitored in a single set of analpses.