Phase separation induced by polymerization of 2-chlorostyrene (2ClS) in polystyrene (PS)/ di-n-butyl phthalate (DBP) mixtures was studied by the time-resolved light scattering (TRLS) technique and scanning electron microscopy. Measurements were performed at a fixed composition of 2ClS:PS::PS : DBP = 45:45:10 by weight in a temperature range from 110 to 180 degrees C that was above the glass transition temperature of the product. Conversion of 2ClS was estimated by gel permeation chromatography. Two distinct time regions were observed in time dependence of scattered light intensity I-m and conversion of 2ClS. In the first region, I-m and conversion increased rapidly, while in the second region, conversion that had reached about 80% changed very little and the growth rate of I-m became much smaller than that in the first region. The paly(2-chlorostyrene)-rich phase formed droplets, and droplets of relatively narrow distribution coexisted with much smaller droplets in an early period. Nonspherical domains mere formed by coalescence of droplets. At lower temperatures, it was observed for the first time that the droplet domain structure transformed into cocontinuous domains. This morphological change occurred while the conversion changed very little. Acceleration of the phase separation rate associated with the morphological change was observed by TRLS. At higher temperatures, coalescence of droplets occurred only in a very early period, and droplets coarsened without forming cocontinuous domains in the later period. The mechanism of the polymerization-induced phase separation of this system is discussed.