The miscibility and crystallization behavior of the solution-blended lightly sulfonated poly(phenylene oxide) (SPPO)/poly( styrene-co-4-vinylpyridine) (PSVP) blend were investigated by conventional and modulated differential scanning calorimetry (MDSC). It was found that the original blend film is actually composed of a crystalline SPPO phase and a noncrystalline compatible SPPO-PSVP phase. The original phase-segregated structure will evolve to a noncrystalline homogenous structure by subsequent high temperature annealing. The resulting good miscibility was attributed to two aspects: one is that the SPPO crystalline structure could be destroyed as annealing temperature is high enough; the other is that the acid-base interaction between the sulfonic group of SPPO and the pyridine ring of PSVP could promote mixing of different components effectively. And such acid-base interaction was demonstrated by C-1 NMR spectra.