Secondary Ion Mass Spectrometry (SIMS) is widely known as highly sensitive a surface analysis technique. Efforts for quantification have been hindered, however, by the presence of matrix effects. Here we describe a new technique for the quantitative analysis of AlxGa1-xAs. Instead of Al+, Ga+, As+ ions, CsX+ ions (X=Al, Ga, As) have been detected. Intensity of these molecular ions appears to be much less affected by matrix effects. We have successfully accomplished the compositional analysis with standard deviation better than 2 percent.