The use of Raman spectrometry for the precise determination of the mole fraction in semiconductor solid solutions is presented. It is shown that if the frequency difference between the modes related to the two binaries involved is used, the scattering of the results due to various experimental conditions (temperature, spectrometer calibration, ...) is avoided. In addition, the method is rapid, nondestructive, reliable, and allows to probe the composition at different depths into the layer.