A thermoset/thermoplastic blend, triglycidyl p-aminophenol (epoxy)/poly(ether sulfone) (PES) was cured using the curing agent 4,4’-diaminodiphenylsulfone, and the structure formation during curl was investigated by time-resolved light scattering. After a certain time lag the scattering peak appeared and its intensity increased with cure time, suggesting the development of reaction-induced spinodal decomposition. Then, however, the intensity started to decrease and eventually reached zero. The intensity decay appears to suggest phase dissolution at the late stage of curing; this was not the case but just an artefact. The apparent phase dissolution was shown to be caused by a large change in the refractive index of the epoxy component with cure reaction, rendering an accidental matching of its refractive index with that of PES. This was confirmed by supplementary experiments.