The objective of this research was to develop resin systems that could be used in structural RRIM composites. The synthetic scheme selected allowed for the control of key physical properties, such as heat deflection temperature (HDT), by varying monomer ratios. A reasonable rationale for the relationship of stoichiometry to physical properties is controlled crosslink density. The system is compatible with both continuous strand glass mat and unidirectional knitted glass. The type of glass and the polymer-to-glass ratio strongly influence both modulus and strength (flexural and tensile) along with HDT and notched Izod impact. Laboratory plaques (12×18 inches) made using a vacuum-assisted, end-gated mold, produced panels with a tensile modulus in excess of 3,000,000 PSI when a combination of unidirectional and continuous strand mat reinforcement (54% glass) was selected. In conclusion, modified unsaturated resins can be used to produce structural RRIM composite parts which are strong, tough, and heat resistant.