Ga0.51In0.49P layers were grown by organometallic vapor phase epitaxy on grooved GaAs(001) substrates misoriented by 9-degrees. The formation of adjacent ordered and disordered domains has been observed on such substrates. The combination of both Raman and cathodoluminescence measurements allows for an independent determination of composition by analyzing the frequency positions of the optical phonon modes (with an accuracy of 1%) and of the band gap from the luminescence at the same position on the sample. Furthermore, ordered and disordered domains can be clearly identified from their different polarization selection rules observed in the Raman spectra. The cathodoluminescence spectra show shifts of the luminescence maxima of up to 100 meV for ordered and disordered domains of the same composition.