The optical absorption of syntheticReS2andReSe2single crystals is reported over a temperature range from 25 to 300 K. Analysis reveals that the absorption edges ofReS2andReSe2are indirect allowed transitions. The indirect band gaps at various temperatures are determined and their temperature dependence is analyzed by the Varshni equation [Physica34, 149 (1967)] and an empirical expression proposed by O’Donnel and Chen [Appl. Phys. Lett.58, 2924 (1991)]. The parameters that describe the temperature dependence of energy gaps of these two materials are evaluated and discussed. ©1997 American Institute of Physics.