In order to obtain the optimum performance for the electron cyclotron resonance/reactive ion beam etching (ECR‐RIBE) reactor, we have examined the effect of Cl2 flow rate on Cl concentration by utilizing the optical emission peak intensity from Cl as the index of its concentration. The peak intensity observed in the reactor not containing GaAs substrate decreases monotonically as the flow rate increases. However, when the GaAs substrate is introduced into the reactor, we observe the intensity decrease drastically at lower Cl2 flow rates. At the same time, strong emission peaks from Ga are observed, and their intensity also shows flow rate dependency. These results can be explained using a simple model that divides the ECR‐RIBE reactor into two continuous stirred tank reactors (CSTRs). We believe that this model and the kinetic constants obtained here can contribute to the efficient optimization of the ECR‐RIBE process. © 1995 American Institute of Physics.