The effect of diffusion control-induced unequal reactivities of functional groups on the gelation of cyanate ester resins was analyzed. By introducing differentiation into the recursive approach, a gelation model was developed to calculate the gel points of systems under diffusional limitations. Modeling results for a Zn-catalyzed cyanate resin show that the unequal reactivities have a significant effect on the gel point and the gelation is delayed to a higher conversion. However, other factors, which may include localized reactions and cycle formation, also contribute to the delay of gelation.