Abstract The curing processes of three types of aliphatic polyurethanes (PUs) were monitored through measuring changes in the IR absorption of the isocyanate group. Two exponential fittings were used to present the decrease of the absorption band of the isocyanate group as a function of the curing time. The curing process contained chemically and diffusionally controlled periods. According to a cross-linking kinetics analysis, one of the PUs followed a first-order, and two others a second-order reaction. The swelling behaviour of the aliphatic PUs was described with a gel fraction and the degree of the cross-linking after the samples cured.