Mechanical and morphological behavior of polystyrene (PS) based compatible polymer blend systems were studied using a tensile tester and scanning electron and optical microscopes. Four different binary compatible blend systems were employed and characterized : PS and poly (2,6-dimethyl 1,4-phenylene oxide) (PPO), PS and poly(vinylmethylether)(PVME), PS and poly(alpha-methyl styrene)(P alpha MS), and PPO and P alpha MS. The compositional dependence of the mechanical properties showed a synergistic effect with respect to modulus, but a negative deviation from the rule of mixtures relationship for strain at break. From the scanning electron microscope (SEM) observations, a deformation mode transition from crazing to crazing and shear banding occurs at similar to 25 wt% PPO in the PS/PPO blends, as indicated by the patch and river patterns above this composition. In the PS/PVME blends, a similar transition was observed at >10 wt% PVME. The PS/P alpha MS blends showed brittle fracture regardless of composition. The PPO/P alpha MS blends showed a brittle fracture for a P alpha MS content >25 wt%. Optical microscope (OM) observations showed that blending of PS/PPO and PS/PVME resulted in a decrease of craze density and length as the PPO and PVME content was increased. PS/P alpha MS and PPO/P alpha MS blends showed few crazes, all of which were localized near the fracture surface. The mechanical and morphological behavior can be explained using models of intermolecular interactions and entanglement density in compatible blends, respectively. Overall the mechanical property and the consequent morphological behavior were similar to the effect of antiplasticization.