Photoluminescence (PL) measurements on Ga0.5 In0.5 P samples grown by organometallic vapor phase epitaxy (OMVPE) on GaAs substrates at various growth temperatures (Tg=600, 650 and 700°C) have been made as a function of pressure up to about 4.5 GPa at 77 K. The PL spectrum shows a rapid shift towards higher energies with pressure, indicating a increase in the direct band gap. In the pressure range 0-3.5 GPa the PL peak energy increases sublinearly with pressure, and a quadratic fit of the pressure dependence was made. The linear pressure coefficient so obtained at low Tg (600°C) is larger than at high Tg (700°C). In addition, at a certain pressure 3.6-4.0 GPa this shift of PL spectra saturates, and then the peak energy shows a tendency to decrease with pressure to about 4.5 GPa at which the emission disappears. These results are explained in terms of repulsion between Γ-folded states in the CuPt-type ordered structure of GaInP alloys.