The refractive index vs wavelength of LiNbO3has been measured between 0.42 &mgr; and 4.0 &mgr; at 25°C and 80°C. The measured values have been expanded in a three‐term polynomial by a least‐squares best fit and the temperature dependence of the polynomial coefficients determined. The results are used to calculate the phase‐matching temperatures normal to the optic axis in nonlinear optical second‐harmonic generation and parametric experiments. The calculated results are compared with experimentally measured values.