Phase separation during curing reaction in the system of Novolac/poly(ethylene-co-vinyl acetate) (EVA) loaded with hexamethylenetetramine (Hexa) was investigated by time-resolved light scattering (TRLS), optical microscopy and scanning electron micrograph (SEM). Novolac/EVA mixtures showed a lower critical solution temperature (LCST) phase diagram with a high asymmetry in which the immiscible region emerges at high temperature and high Novolac contents. Hydrogen-bonding interaction between Novolac and EVA was verified by FTi.r. measurement as well. As the curing reaction proceeded, an initially homogeneous Novolac/EVA/Hexa mixture underwent phase separation via spinodal decomposition (SD), which was induced by depression of LCST with the increase of the molecular weight of Novolac. A regular and cocontinuous phase-separated structure was finally formed for the cured Novolac/EVA mixture. Time-resolved light scattering investigation showed that the process of SD during curing was different from that in ordinary thermally induced SD, i.e. q(m), the wavenumber q at the peak position of the scattering profile, increases with time in the early stage of curing. The structure formation process was discussed in terms of the SD mechanism based on the investigation of TRLS and SEM observation. The scaling structure functions analysis and SEM observation show that the final morphology of the cured mixture may be yielded through the route of the cluster to the cocontinuous structure.