Donor‐related deep levels in Se‐doped In0.5(Ga1−xAlx)0.5P (x=0.0–1.0) have been studied by DLTS,C–Vmeasurement, and photocapacitance techniques. Two donor‐related deep levels were found. The concentrations linearly increased with the donor concentration, and strongly depended on the alloy composition. These levels were found to be dominant donors in the composition range fromx=0.3 tox=1.0. They had 0.20 and 0.29 eV activation energies for electron thermal emission, 0.08 and 0.11 eV energies for electron thermal capture, and they had 0.7 and 1.1 eV energies for electron optical emission, respectively. Internally consistent configuration coordinate diagrams were proposed for these levels, suggesting that both levels are similar toDXcenters commonly found inn‐AlGaAs.