The electron capture cross section of sensitizing centers for photoconductivity has been measured as a function of temperature between 77° and 350°K in CdS, Cd(SSe), CdSe, GaAs, and InP crystals. The value of the cross section is obtained from the measurement of the minimum number of quenching photons per second required to initiate optical quenching, at a temperature below that at which thermal quenching occurs. Forty‐one determinations of cross‐section on nine different crystalline samples yield values between 10−20and 10−22cm2with only slight temperature dependence. A consideration of the various models to account for such small capture cross sections without appreciable temperature dependence suggests that the cross section is associated with radiative capture by a neutral center.