The compatibility and phase morphology of poly(phenylene oxide) (PPO) multicomponent blends with poly(ethylene terephthalate) (PET) and polystyrene (PS) were studied using differential-scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM) methods. The effect of glycidyl methacrylate-styrene copolymer (GMS), as a compatibilizer, on the morphology of the PPO blends has also been studied in detail. The influence of the molecular weight of PET and the synergetic effect of the compatibilizers of GMS and phenoxy (PN) on the morphology were examined. The DSC and DMA results show that two distinct glass transitions corresponding to PET and PPO existed; however, the T-g of PPO shifts toward lower temperature region due to the addition of GMS and PS. The SEM results reveal that PET component exists as dispersed phases in the PPO matrix, while PS is miscible in the PPO matrix. A significant improvement of the compatibility was achieved for the PPO multicomponent blends because of the synergetic effect of GMS and phenoxy.