Enthalpiemetric systems have been described for monitoring the enzyme reactions in organic solvents. The thermal capacity of organic solvents is much lower than that of water and this increases the sensitivity of the enzyme substrate detection. This was confirmed by the determination of the activity of lipoprotein lipase in the binary system water—toluene, of the peroxidase activity in toluene and that of chymotrypsin in a 10 % aqueous dimethylformamide. In the latter case, the reaction of esterification catalyzed by chymotrypsin in such a medium is endothermic so that the decrease in the temperature was monitored contrary to the majority of other enzymatic reactions.