Biochemistry Department, School of Tropical Medicine, Calcutta-12 Manuscript received 30 January 1974; revised 30 November 1974; accepted 22 January 1975 Human serum esterase was utilised to hydrolyse phenyl, \(m\)-methylphenyl, p-methylphenyi and p-chlorophenyl acetates at the optimum pH determining kinetically for each the Michaelis constant, Km whose reciprocal, the affinity constant Kα showed on correlation with Hamnzett's substituent constants, that the enzyme-substrate complex formation is facilitated by electron attracting substituents. Inhibition studies indicated the participation of a divalent metal and non•participation of any c-amino group of lysine or any thiol group from the enzyme molecule. The mechanism for complex formation postulates the metal bound to the enzyme with two valen­cies and to the two oxygen atoms of the substrate with two other valencies. The breakdown of the complex is considered to be by an attack of a hydroxyl ion of the medium upon the acyl carbon whose fission with the bridge oxygen ultimately produced phenol, acetic acid and free enzyme.