The refractive index of GaN and its temperature dependence have been measured using plane-parallel platelets with E ⊥ c. In this direction at 300 K the index is 2.67 at 3.38 eV, 2.33 at 1.0 eV, and has an extrapolated value of 2.29 ± 0.005 at 0 eV. The difference in the refractive indices for E ⊥ c and E ‖ c is 1.5 ± 0.2% at 5000 A. A Debye temperature of about 600 K is estimated from the measured temperature dependence. The diffuse reflexion spectrum of GaN powder has been measured and the temperature dependence of the absorption edge is plotted.