Epitaxy of III–V semiconductor layers from gaseous sources metalorganic chemical vapor depsoition and metalorganic molecular‐beam epitaxy is essentially determined by surface reactions of metalorganic components on the semiconductor surface. The present paper deals with the decomposition of trimethylgallium [TMGa=Ga(CH3)3] and trimethylarsenic [TMAs=As(CH3)3] on Si(111) and Si(100) surfaces. High‐resolution electron energy‐loss spectroscopy (HREELS) is used to identify the adsorbed surface species by means of its vibrational spectrum. Adsorption of TMGa and TMAs is performed at 300 K and the surface reaction, in particular its dependence on temperature is studied by measuring HREELS spectra after each step. A comparison with thermal desorbtion data is given in some cases. TMGa and TMAs exhibit a different decomposition pattern and the reaction of TMAs is dependent on the orientation of the Si surface.