Enthalpies and entropies of fusion have been determined for two series of semicrystalline polyurethanes. The H series is based on 1,6-hexamethylene diisocyanate and various α, ω-diols containing from 2 to 10 methylene units. The DP series is based on 4,4′-methylene diphenyl diisocyanate and various α, ω-diols containing from 2 to 10 methylene units. Differential Scanning Calorimetry (DSC) was used in conjunction with X-ray analysis (X-ray method) and low molecular-weight diluents (diluent method) to determine the enthalpies of fusion. Agreement between the X-ray method and the diluent method was good.Differences between the enthalpies and entropies of fusion of the H and DP series could be largely attributed to differences in intermolecular hydrogen bonding in the melts of the two series. On the other hand, differences in the entropies of fusion between the H series and corresponding polyamides arise largely from differences in backbone-chain stiffness between the two series.