The properties of deep traps in undoped, Si‐, S‐, Se‐, and Te‐doped In0.5 Ga0.5 P layers grown on GaAs substrates by liquid phase epitaxy are investigated by deep level transient spectroscopy, thermally stimulated capacitance, and Hall measurements. Only one kind of deep trap is observed in undoped layer. Among the doped layers, it is only in the S‐doped layer that the deep trap concentration is increased and the persistent photoconductivity is observed. Furthermore, the deep trap properties in undoped and S‐doped layers are nearly the same. Considering the amount of residual S atom in the undoped layer, it is suggested that the deep trap in the undoped layer may result from the residual S impurity. ©1995 American Institute of Physics.