Particulate polymer composites of lsophthalate based unsaturated polyester resin filled with different concentrations of lead monoxide were prepared and investigated for physical, thermal, mechanical, and gamma radiation shielding characteristics. The results of thermomechanical studies have been discussed in this article. The composites were investigated to understand the effect of filler on glass transition temperature (T-g) and thermal expansion coefficients (alpha) at various temperatures under cyclic heating and cooling conditions. The results show that T-g values of composites appear to decrease slightly with increasing filler content. The composites were observed to exhibit three phases of thermal expansion during first heating and cooling cycles where as they exhibit only two phases during second heating cycle. An anomalous expansion of the composites was observed in the temperature range from 60 degrees C to 110 degrees C only during first heating and it disappears in subsequent heating cycles. Further, all the composites were observed to exhibit a dimensional contraction only during first heating and cooling cycles. The cyclic heating and cooling technique using thermo-mechanical analyzer was observed to be very efficient in determining actual values of T-g and also in determining the post curing temperatures of the polymer and its composites. (C) 2010 Wiley Periodicals, Inc. J April Polym Sci 117: 3623-3629, 2010