Polystyrene (PS), after polyolefins, is the most widespread polymer both in industry and daily life, successfully replacing some natural raw materials. In this study, as a serial of our previous study, modified polystyrene containing acetyl group (CH(3)CO-) was used as a raw material and condensation reaction was performed in basic medium with aliphatic saturated and unsaturated aldehydes. The structures of modified polystyrenes with unsaturated keto-groups were investigated by chemical and spectral (FTIR vs. (1)H NMR) analyses. The effect of the functional groups bound to the structure of the polymer on physicomechanical and thermal properties of modified polystyrene was investigated. As a result, condensation products of modified polystyrenes with crotonaldehyde and cinnamaldehyde had higher physicomechanical properties and were more stable against the thermal destruction at high temperatures. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1193-1202, 2011