The defect properties of Si-doped Ga0.52In0.48P layers grown by solid source molecular beam epitaxy (SSMBE) have been studied by deep-level transient spectroscopy (DLTS), temperature-dependent electron concentration, and capacitance-temperature (C-T) measurements. The persistent photoconductivity (PPC) effect was observed in highly Si-doped samples. The same samples showed a linear relationship, and changes of almost the same order of magnitude between the trap concentration and electron concentration. There is evidence to suggest that these donor-related levels can be attributed to the presence of DX centers. This defect exhibits ionization energies from similar to 358 to 435 meV, but is located at only similar to 11-13 meV below the conduction band. The energy level, capture barrier, and ionization energy characteristics are typical of those for a donor-related DX center. The electric field effect (Poole-Frenkel effect) on the emission rate strongly suggests that the defect is shallow.