The temperature and compositional dependences of the energy gap are determined for the alloy semiconductor mercury‐cadmium telluride (Hg1−xCdxTe). The cutoff wavelength was measured on photoconductive and photovoltaic infrared detectors where 0.17<x<0.60 over a temperature range 20<T< 300°K. Spectral response curves are shown to indicate the precision of the method. The temperature and compositional dependences of the energy gap are shown and the empirical expressionEg(eV) =1.59x−0.25+5.233(10−4)T(1−2.08x)+0.327x3is deduced.