The preparation of novel polyurethanes, poly(N-acylurethane)s, by polyaddition of bis(N-acyl isocyanate)s with diols under mild conditions is described. Bis(N-acyl isocyanate)s (1a-c) were derived from the reaction of the corresponding diamides with oxalyl chloride in good yields. The polyaddition of 1 with aliphatic and aromatic diols (2 and 3) was performed at room temperature in dimethylacetamide without catalyst to quantitatively afford the corresponding poly(N-acylurethane)s (4 and 5, respectively). The high reactivity of the N-acyl isocyanate group compared to the unsubstituted isocyanate was demonstrated by the fact that bis(N-acyl isocyanate)s readily form a polymer with 2-n-dodecyl-1,3-propanediol while m-phenylene diisocyanate does not form a polymer under the same conditions. Poly(N-acylurethane) with a polymeric diol segment was prepared similarly using hydroxy-terminated poly(tetrahydrofuran). Solubility, molecular weight, glass transition temperature (T(g)), and 10 % weight loss temperature (T(d)) of the polymers were evaluated.