A systematic study has been made of the deep level introduced into GaAs1−xPxalloy material by S doping. Conclusive documentation of the linear relationship between S concentration and the deep‐level trap concentration is presented forx?0.4. The dynamic properties of this trap measured atx=0.38 show a temperature activation energy for emission and electron capture of 0.35 and 0.15±0.03 eV, respectively. It exhibits a very large hole‐capture cross section at a site occupied by an electron. A very sharp drop in the ability to populate the trap belowx=0.35 is consistent with the trap being energetically tied to theXband minimum and having a depth of 0.17 eV below this band minimum. The dependence of the thermal‐emission activation energy on alloy composition is ascribed to the necessity to capture electrons from the indirect minima belowx?0.4. Photoionization experiments indicate that a much greater photoenergy than the thermal energy is required to ionize an electron trapped at this site (1.1 compared to 0.17 eV). Electron capture followed by a large lattice relaxation is consistent with the data and the phenomenon of persistent photoconductivity at low temperature associated with the S center. The S center in GaAs1−xPxprovides an efficient nonradiative recombination path for minority carriers.