The causes of void content and its resulting effect on the material properties of structural reaction injection molded (SRIM) composites were investigated. Plaques were molded under different combinations of five two-level factors, and the resulting effect of each factor on void content was determined. The geometry of the glass reinforcement had the largest effect, as woven mats caused double the void content of random mats. Evacuating the mold cavity proved to greatly reduce void content as well. Varying the injection rate and shot size had little effect. Material property testing was conducted on groups of specimens with similar void content. Tension testing revealed no decrease in strength with increasing void content. One million cycles of tension-tension fatigue at 20% of the ultimate strength caused no decrease in strength at any void content, while fatigue at 30% caused premature failure. Impact testing showed that only very high void contents had a substantial effect on performance. Environmental conditioning showed moisture absorption proportional to void content and furthermore significantly reduced the residual tensile strength after a single thermal cycle to 0-degrees-C. These results suggest that void content in conjunction with a weak interface may have been responsible for limiting material properties.