Monoacylated mesitylene is industrially an important precursor used. in fine chemical and intermediate industries. The current processes use highly polluting and hazardous catalysts. In the current work, selective synthesis of 2',4',6'-trimethyl acetophenone (monoacylated mesitylene) from 1,3,5-trimethyl benzene (mesitylene) and acetyl chloride was studied by using K-10 montomorillonite clay based heteropolyacids such as dodecatungstophosphoric acid (DTP) and dodecatungstosilicic acid (DTS). Acetyl chloride was found to be a better acylating agent as compared to acetic anhydride and acetic acid. 20% w/w DTP/K10 was found to be the most active and 100% selective catalyst for monoacylation in comparison with others, either supported or unsupported. A complete theoretical and experimental analysis is presented. The reaction follows the Eley-Rideal type of mechanism with weak adsorption of the acylating species. The energy of activation was found to be 18 kcal mol(-1), indicating that the reaction is intrinsically kinetically controlled.