In this study, three polystyrenes having different molecular weights have been chemically modified with maleic anhydride by using a cationic catalyst of Lewis acid type BF3O(C2H5)2 (BF3) in chloroform. The effects of polystyrene (PS) and maleic anhydride (MA) concentrations, temperature and solvent volume on the chemical modification have been studied. IR spectra of modified polystyrene existing at the end of the chemical modification have been taken to show the presence of carboxyl group fixed to the aromatic ring of the polystyrene. Reaction rates have been calculated by determining the amount of carboxyl group using volumetric method. Some physical parameters, such as rate constants, activation energy of the reactions for chemical modification of polystyrene have also been calculated. © 2006 Elsevier B.V. All rights reserved.