Single-phase mixtures of phenolic derivatives with poly(styrene-ran-acrylonitrile) copolymers (SAN) were cured in the presence of hexamethylenetetramine, and the effect of the acrylonitrile (AN) content in SAN on the phase separation and reaction kinetics during curing were investigated by light scattering, differential scanning calorimetry (DSC), and optical microscopy (OM). DSC measurements revealed that the reaction rate does not have a significant effect on the AN content in SAN but depended on the fraction of multifunctional phenols in total phenols. The time variation of the light scattering profile during cure demonstrated the characteristic feature for spinodal decomposition. OM observation revealed that a co-continuous two-phase structure appeared after a certain time lag and coarsened to a spherical domain structure consisting of phenolic-rich particles dispersed in the SAN-rich matrix. The periodic distance, Lambda(m) in the phase-separated structure changed with curing time, as follows. The higher the miscibility between phenolic resin and SAN, the higher the fraction of multifunctional phenols in total phenols, the higher the SAN composition, and the lower the cure temperature, the shorter the Lambda(m) was.