Reinforced reaction injection molding, RRIM, using preplaced glass‐fiber mats is an attractive method for production of composite materials. The effect of the glass‐fiber mat on the properties of a styrene‐dimethacrylate copolymer (SDM) is analyzed. Thermal conductivity as a function of porosity was measured. A simple method to obtain the thermal coefficient of linear expansion using the Rheometrics System Four is presented. Dynamic mechanical properties were measured using torsion and three‐point bending of rectangular bars. Adding the glass‐fiber mat increased the storage modulus (G′) by a factor of 5 below Tg and 100 above it. Empirical correlations for the composite modulus as a function of glass‐fiber content below and above Tg are presented. The increase in glass‐transition temperature with increasing glass‐fiber content is discussed.