In this study, optical and structural properties of InGaP/GaAs epitaxial heterostructures are investigated as a function of growth temperature and substrate orientation (2 and 10 degrees). Both ordered and disordered InGaP layers were grown by metal organic vapour phase epitaxy (MOVPE). The complex refractive index both above and below the fundamental band gap has been determined using spectroscopic ellipsometry (SE). In particular an InGaP/GaAs intermixing layer, in addition to the InGaP oxide overlayer was identified and introduced in order to improve the accuracy of the extracted refractive index. Variations in Raman TO,, mode and photoluminescence (PL) emission efficiencies show different degrees of ordering, which are correlated to different PL peak energies or bandgaps extracted from SE data. Misfit strain and lattice mismatch are also investigated.