The dynamic of automatic system control in conditions of absence of the operate measurement of exit parameter by utilizing a intermediary parameter as may be the control system of gallium arsenide epitaxial layers impurity concentration in alloying process, obtained by transport of reactions in the Ga-AsCl3-H2 gas system, was investigated. Because of purity reasons in the reactor cannot be intro-duced sensors for inside parameter measurement. The au-tomat control system is working by the alloying substance (zinc, tellurium) temperature measuring, which supplies the vapour flow source at the reactor entrance. The vapour flow, generated in the chamber of alloying source and transported it in the reactor by hydrogen flow, is arrived in the growing zone of the reactor with 2-6 min of delay, because the reactor capacity is bigger than alloying chamber capacity. Optimization parameters of the technological process are the time of the reactor filling with vapour and the time of its evacuation after the vapour flow is interrupted. Those parameters are in direct relation with the capacity of the vapour source and they need to be minimized by a forceful overshooting of alloy source temperature.