Intrinsic fluorescence during the reactions between methylene 4,4’-diphenyl diisocyanate (MDI) and l-butanol or hydroxy-terminated poly(propylene oxide) was investigated to characterize a model urethane reaction and polyurethane formation both in dilute solution and in bulk. Fluorescence intensity was found to increase about 87 times when MDI was converted to diurethane, with the emission maximum around 315 nm. The rate constants and the activation energies for the model reaction and polyurethane formation were obtained from the fluorescence results and compared with the IR results. The correlation between the fluorescence intensity and the extent of the reaction determined by IR has been established.