Department of Chemistry, Annamalai University, Annamalainagar-608 002 Manuscript received 2 May 1981, accepted 28 January 1983 The rates of reduction of a number of substituted 4-methylthioacetophenones by sodium borohydride in isopropanol medium have been studied. The observed values of 3-methyl and 3-halogeno-4-methylthioacetophenones record significant deviation from the values calculated on the basis of additivity principle. The observed rate constants are lower than the predicted values. This indicates that the mesomeric interaction between the methylthio and acetyl groups is enhanced when there is a substituent, \(ortho\) to the methylthio group. " In the case of 3-halogeno compounds, the percentage of enhancement increased as the bulk of the halogen increases. The expected steric inhibition of resonance is observed in the case of 3,5-dimethyl-4- methylthioacetophenone.