Using Raman scattering we have measured the longitudinal optic phonon energies of Ga1-xInxAs layers with 0.48=x=0.55 grown on InP substrates. In order to interpret these data successfully it was necessary to consider the effect of the strain induced by the lattice mismatch. Existing theory has been extended to consider the case of alloy pseudomorphic layers and is approximated to a linear form. This theory is also applied to Ga1-xInxAs on GaAs and compared with previously published data with 0=x=0.2. Close agreement between theory and experimental data is found indicating that the Raman technique, combined with this theory, can be used to measure the alloy composition of pseudomorphic strained layers of diamond or zinc blende structure accurately without the need for detailed experimental calibration. It is suggested that this method could be particularly useful for determining the composition of alloy layers in thin layer pseudomorphic Ga1-xInxAs/InP heterostructures.