Syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends, miscible in the melt state, were crystallized from the melt and the quenched state at different temperatures. The effect of the crystallization temperature on the phase behavior of the blends and the polymorphic changes in sPS was investigated by dynamic mechanical analysis (DMA), wide-angle X-ray diffraction (WAXD), and density measurements. In most blends, the crystallization of sPS induced segregation into two homogeneous amorphous phases of different compositions. The temperatures of the DMA relaxations of the neat homopolymers and crystallized blends were fit by the Gordon-Taylor relation to calculate the compositions of these phases. In melt-crystallized blends, with slower crystallization, the major amorphous phase became sPS-rich, whereas the minor phase became PPO-rich. These major and minor amorphous phases could be tentatively assigned to interfibrillar and interlamellar regions, respectively. In cold-crystallized blends, slower crystallization decreased the sPS concentration in both phases, and the scale of segregation was much smaller. WAXD studies and density measurements indicated a complex polymorphic behavior of sPS after it was blended with PPO. (C) 2003 Wiley Periodicals, Inc.