Optical transmission measurements were performed on GaN bulk single-crystal platelet at temperatures between 293 and 1237 K. The energy bandgaps were determined from the corresponding optical absorption spectra. The bandgaps can be fit well as a function of temperature using the Varshni expression as Eg(eV) = 3.556 - 9.9 x 10(-4) T-2 /(T + 600). The shapes of the measured absorption edges were found to be dependent on the thermal treatments of the sample. (C) 2002 Published by Elsevier Science B.V.