A resin containing fumarate isophthalate and propylene glycol structural units was cured with styrene and characterized by C-13 solid-state nuclear magnetic resonance spectroscopy. The degree of residual polyester unsaturation was estimated from relative areas of peaks at 172ppm (carboxylic carbon in saturated structural units derived from fumarate) and 165ppm (carboxylic carbon in isophthalate and unreacted fumarate structural units). Decreasing the styrene content below 47% by weight resulted in residual polyester unsaturation. Results were interpreted in terms of a statistical model for copolymerization, providing evidence for low reactivity of fumarate structural units relative to diethyl fumarate and drawing attention to differences in reactivity between the resin system used in the present work and those used in earlier studies.