A molecular beam/surface scattering experiment in an ultrahigh vacuum is conceptually a simulation of a CVD reactor without the interference from gas phase and wall reactions. The surface chemistry can be studied in real-time during the deposition reaction at the desired temperature. In our experiment, we used pulsed molecular beams of the reactants and a mass spectrometer to monitor In real-time the reaction products evolving from the substrate surface. With this arrangement, the reaction probability of the molecules can readily be determined by measuring the unreacted fraction of the molecular beam. The reaction pathways can be deduced from the Identification of the reaction products, while their time-evolutions give the kinetic parameters. We shall illustrate this technique by our study on the reactions of trimethylgallium and triethylgallium on GaAs as related to the metalorganic CVD and atomic layer epitaxy of GaAs.