Using Bube’s formulation of the sensitization process in photoconductors a particular exchange mechanism is proposed that associates a shallow trap (excited state) and a recombination level (fundamental state). Expressions are deduced describing the photocurrent behavior as a function of temperature and illumination on both sides of the continuous thermal quenching zone; explicit formulations are also deduced for the upper and lower break points in the lux‐ampere characteristics. Such a phenomenological model is applied to photoconduction data on stannic oxide high‐resistivity samples. Optical quenching and pulsed photoconductivity are also emphasized in order to propose a consistent scheme for the localized levels in the forbidden energy band. It is shown from this work that localized transitions occur between three possible recombination states and a common excited state working with the conduction band as a shallow trap; associated lifetimes are measured.