The diffusion phenomena, including the molecular diffusion processes and the effect of diffusion on the cure kinetics and molecular size distribution, during the curing of a zinc-catalysed cyanate ester resin have been studied. The average diffusivity, which was estimated using the dielectric analysis-based approach, decreased by several orders of magnitude during the curing process. The molecular size distribution measured with size exclusion chromatography was found to deviate from the mean-held prediction at relatively high conversions. By using the Rabinowitch model, a diffusion-controlled kinetic model was formulated, which adequately predicted the experimental cyanate conversion profiles over the entire range of cure.