The surface of epilayers grown by chemical vapor deposition (CVD) cannot be characterizedinsituby RHEED, LEED, or Auger spectroscopy methods used for molecular beam epitaxy. But, unlike the epilayers grown by molecular beam epitaxy, those grown by CVD method have their crystallographic and electric properties controlled by the characteristics of a 4–20‐μm‐thick motionless gas layer called boundary layer. Samples of the boundary layer can be probed as a function of the distance above the growth surface and analyzed by gas phase chromatography in the case of silicon epitaxy in order to measure concentration and concentration gradient of pertinent chemical species close to the growth surface. This analysis allows us to explain the behavior of the growth rate, the kinetics of the incorporation of impurities and the crystallographic quality of eqilayers as a function of the growth parameters. General results can be used to explain most cold wall reactor behavior. GaAs and InP materials grown by the organometallic method are treated as examples.