Polystyrene is, after polyolefins, the most widespread polymer in both industry and everyday life, successfully replacing some raw natural materials. In this study, the chemical modification of polystyrenes of different molecular weights was performed with various functional group modifiers (epichlorohydrin, maleic anhydride, and acetic anhydride) in one stage and in the medium of the cationic catalyst BF3.O(C2H5)(2) according to previous studies. The concentration of the functional groups bonded to the aromatic ring of the polymer as a result of the chemical modification of polystyrenes of different molecular weights depended on the molecular weight of the polymer, and more functional groups were bonded to lower molecular weight polystyrene. The effects of the functional groups bonded to the structure of the polymer on the thermal properties of modified polystyrene were investigated. The polystyrene that was modified by maleic anhydride was more stable against thermal destruction at high temperatures. (c) 2007 Wiley Periodicals, Inc.