Thermal- and impurity-induced order-disorder transformations of Ga0.5In0.5P epilayers grown by metalorganic chemical vapor deposition have been investigated by photoluminescence. The energy gap splitting was observed during the transition from ordering to disordering. This is due to the variation of the valence-band spin-orbital split resulting from the interband reaction. The photoluminescence depth profile of the epilayer (incompletely transferred to disordering) is used to explain the phase transformation. A defect-associated order-disorder transformation model has been proposed.