Montmorillonite clays are reported as efficient, inexpensive, and reusable catalysts for carbon-sulfur bond formation by conjugate addition of thiols to alpha,beta-unsaturated ketones/ester/nitrile. The reaction of aryl and aryl alkyl thiols with cyclic/acyclic alpha,beta-unsaturated ketones/ester afforded excellent yields after 5 min to 20 h. The reaction rate was found to be influenced by the (i) size of the ring in case of cyclic enone, (ii) electronic nature of the thiol, and (iii) presence of aryl/alkyl substituent at the beta position of the acyclic alpha,beta-unsaturated ketone/nitrile. The conjugate addition of thiols took place at faster rates for five-membered and acyclic alpha,beta-unsaturated ketones than the six-membered analogue. Aryl thiols reacted at faster rates than aryl alkyl and alkane thiols and the differential reaction rates were attributed to the relative acidic strength of the thiols. The reaction of alpha,beta-unsaturated ketones having an aryl/alkyl group at the beta-carbon took longer times and higher temperature. The difference in the reactivity between six and five membered enones and various thiols was utilized to demonstrate selective thia-Michael addition reaction during intermolecular competition studies. (c) 2006 Elsevier B.V. All rights reserved.