Specific refractive index increments nu of polyester-based segmented polyurethanes in N,N-dimethyl formamide have been determined, and the quotient d nu/df(d) has been evaluated (where f(d) is the weight faction of hard-segment units). The results are in good agreement with the values calculated from group contributions to the molar refraction, using the Vogel or the Gladstone-Dale equations. The values calculated with the Lorenz-Lorentz equation are too low. A potential explanation of this fact is proposed. The same methods have been applied to reported nu values for polyether-based polyurethanes. An explanation is proposed for differences in d nu/df(d) for polyester- and polyether-based polyurethanes.