Two unsaturated polyesters, one based on phthalic anhydride PEP and the other based on isophthalic acid PEI, were synthesized. The chemical structure of the two polyesters was characterized by IR and 1H and 13C NMR spectroscopy. The effect of styrene concentration on the curing of polyesters was also studied. It has been found that the percent of polyester/styrene (70/30 wt %) gave the highest percent of curing. Different concentrations of carborundum (070 wt %) were used to prepare polyester composites. A comparative study was done on the properties of the two prepared polyesters PEP and PEI and their composites in term of their thermal, mechanical, electrical, and physical properties. The results indicate that the polyester based on isophthalic acid (PEI) and its composites gave higher compressive strength values and lower water absorption than those based on phthalic anhydride (PEP). The presence of carborundum improved the thermal stability than the cured polyesters and electrical properties. (c) 2012 Wiley Periodicals, Inc.