The catalytic activities of chloroperoxidase (CPO) including halogenation, oxidation, and peroxidation were investigated in the presence of ammonium salts: tetramethylammonium bromide (TMABr), tetraethylammonium bromide (TEABr), tetrapropylammonium bromide (TPABr), tetrabutylammonium bromide (TBABr), and cationic surfactants: dodecyltrimethylammonium bromide (DTABr) and cetyltrimethylammonium bromide (CTABr). All the mentioned activities were promoted in most cases. The highest modified activity (ABTS peroxidation) was 18.16 times higher in the presence of TMABr than that in pure buffer. The activity enhancement was strongly dependent on the concentration and the hydrophobic chain length of additives, and the structure of substrates. The kinetic parameters showed that the activation was mainly attributed to an increase in k(cat) due probably to a catalytically favorable conformation of CPO induced by the additives. Moreover, a lower K-m and higher ratio of k(cat)/K-m (specificity constant) was obtained, indicating that both the affinity and specificity of CPO to substrates were improved in the presence of additives. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1024-1028, 2010