Photoconducting properties of GaAs–InAs and GaAs–GaSb alloys were investigated at room and low temperatures. The peaks in the photoresponse were found to vary smoothly across the alloy systems. Band gap values obtained from the spectral sensitivity curves confirmed the parabolic variation of energy gaps as a function of the alloy concentration, as previously found in electrical conductivity and optical absorption measurements. The room temperature detectivity of the samples was between 2 × 105 and 8 × 106 cm (Hz)1/2 W−1 for wavelengths varying from 0.8 to 3.5 μ, depending on alloy composition.