The bimodal phase separation process of a rubber-modified epoxy system, consisting of diglycidyl ether of bisphenol A (DGEBA), and a hydroxyl-terminated butadiene-acrylonitrile random copolymer (HTBN), during curing with tetrahydrophthalic anhydride was studied by time-resolved small-angle light scattering (TRSALS), differential scanning calorimetry (DSC), and digital image analysis (DA). The HTBN/DGEBA mixture reveals an upper critical solution temperature (UCST). At higher curing temperatures, double-peak structure from the matrix was investigated by TRSALS and confirmed by DIA. The special two characteristic size distribution behavior was explained qualitatively by nucleation growth coupled with spinodal decomposition (NGCSD) and the competition between phase separation and polymerization.