Unsatd. polyester resins were reinforced by addn. of hollow glass microspheres, NH4 and melamine phosphates as well as nano particles and studied for viscosity and gelation time. The resulting composite materials showed high flame resistance and good thermal stability as evidenced by detn. of limiting O-2 index and cone calorimetry. The digital micrographs confirmed the good morphol. of the composites and synergistic effects of the additives used.