The basic studies of in situ compatibilization process between polystyrene (PS) and polybutylene terephthalate (PBT) are carried out by adding various functionalized polystyrenes with mono, di, tetra hydroxy functions. The tensile mechanical properties of these in situ compatibilized blends are investigated. It is important to control the thermal degradation of PET during the high temperature processing in order to generate the reaction between the functionalized polystyrene and PET and result in the improvement in their mechanical properties. However, the excess degradation deteriorates the tensile properties of the in situ copolymerized and compatibilized polymer blends. It is found that the usage of Titanium butoxide (TNBT) was effective to promote the thermal degradation of PET and the copolymerization of PET with the functionalized polystyrenes. On the other hand, Triphenyl phosphite (TPP) was used to prevent the excess thermal degradation and induce the possible end capping reaction of PET. The addition of 5% PS-FLC (tetra functionalized polystyrene) in PS/PBT 10/90 blend at 270 degreesC with 1% TPP exhibits the best improvement in the elongation at break among the mono-, di- and tetra-functionalized polystyrenes. The increase of amount of the functionalized polystyrenes into the PS/PBT blends decreased the elongation to break, the yield strength, and the energy to break. More functional groups in the functionalized polystyrenes are more effective in promoting in situ compatibilization. Furthermore, the one-step mixing in which all the ingredients were blended together for 10 min has better elongation to break than the two steps mixing. The detailed characterization of rheological, morphological and mechanical properties of various blends of functionalized PS/PBT and PS/PBT with different functionalized polystyrenes and the additives are described. (C) 2001 Elsevier Science Ltd. All rights reserved.