Copolymer resins were synthesized by the mixed condensation of resorcinol-CH2O, resorcinol-aniline-CH2O, resorcinol-aniline-urea-CH2O, resorcinol-aniline-thiourea-CH2O, and resorcinol-aniline-urea-thiourea-CH2O with a view to develop synthetic resins having improved properties and performance like toughness, mechanical properties, thermal resistance, chemical inertness, and adhesion. The resins were characterized by IR and TG analyses. The thermal analysis showed that the mixed copolymer resins have significantly better thermal stability than that of the simple phenol-CH2O, aniline-CH2O, urea-CH2O, and thiourea-CH2O resins. At 600 degrees C, only a 50% weight loss is observed, and even at 800 degrees C, the weight loss is about 70%. The resins are insoluble in most of the organic solvents including DMSO-d(6). They too are unaffected by diluted HNO3, but dissolve in concentrated HNO3.