The article considers the basic processes occurring in a laboratory reactor during plasmachemical treatment of gallium arsenide (GaAs) in chlorine plasma. The treatment of GaAs has a significant influence both on etching processes of semiconductor material and on the formation of the stationary concentration of chlorine in the reactor. The chlorine atoms act as the main chemically active particle (CAP) in etching GaAs. Along with the processes of chlorine atom formation we deal with the physical processes of their heterogeneous recombination on the inner surface of the glass reactor and on GaAs samples. The work demonstrates experimentally the exaggeration of the rate constants of interaction of chlorine atoms with GaAs without taking into account the stages of heterogeneous recombination of chlorine atoms on the surfaces bounding the plasma zone. We carry out gallium arsenide etching in a flow reactor at low plasma gas flow rates and the constant total pressure. In order to study the diagnostic purposes and experimental determination of rate constants of target processes of chemical and heterogeneous CAP recombination directly in the zone of the positive column of the glow discharge in chlorine we use a relaxation pulse technique.