Thermoplastic, light-stable polyurethanes have been prepared by the interfacial poly-condensation of secondary diamines with the bischloroformates of polyester diols derived from ε-caprolactones. A straight-line relationship was obtained from a logarithmic plot of inherent viscosity versus Mn. The effect of some structural variations on the properties of the polyurethanes is discussed. These variations included the molecular weight of the polyester diol, the initiator used in preparing the polyester diol, the introduction of a methyl substitutent on the caprolactone ring, and the structure of the diamine. Polyurethanes prepared from polyester diols of Mn from 2000–4000 are opaque, stiff materials which undergo a melting transition at 40–50°C. characteristic of the polyester component. Polyurethanes which are flexible and translucent at room temperature are obtained either by using a low molecular weight polyester diol or a copolyester diol containing > 25% methyl ε-caprolactone. The polyurethanes described are soluble in several common solvents and can be cold-drawn to produce material of high tensile strength. Thermal studies using DTA and TGA Show an exothermic reaction starting above 300°C. Rapid weight loss beginning at 300–330°C. and leveling off near 500°C. accounted for 85–90% of the total sample and is attributed to decomposition of the polyester Chains.