Phase separation kinetics and morphology were studied for polystyrene/poly(2-chlorostyrene) blends containing a small amount of di-n-butyl phthalate over a wide composition range by using the time-resolved light scattering technique and electron microscopy. Cocontinuous domains and isolated spherical domains (droplets) were observed to coexist in the critical to off-critical crossover composition region : It was found that the phase-volume fraction was not the only factor determining the crossover compositions. Two maxima corresponding to the scattering from cocontinuous domains and droplets respectively were observed in the scattered light intensity profile. Because of a large difference in the coarsening rate between the two domain structures, only the maximum corresponding to the cocontinuous domains was initially observed. The wavelengths at which the two maxima were located could be expressed by power functions of the phase separation time, and the exponent for each maximum had the same value as in the case where the corresponding domain structure existed separately. This indicated that the coarsening mechanisms of the cocontinuous domains and droplets were scarcely changed by coexistence with the other domain structure.