Mercuric bromide (HgBr2) and mercuric bromide iodide (HgBrI) are layered semiconducting compounds exhibiting promising features and can be considered as candidates for nuclear radiation detector applications that operate at room temperature. In the present work, the optical transmission spectra of HgBr2 and HgBrI single crystals were measured in the fundamental energy gap region, in the 11-310 K temperature range, and the absorption spectra were obtained. The absorption coefficient exhibited exponential behavior following Urbach＇s rule, for both compounds. The values of the energy gap at several temperatures as well as the temperature dependence of these compounds were determined, and a comparison was made with the corresponding features of HgI2.