We have investigated the production and loss kinetics of SiClX radicals during silicon gate etching processes in HBr/Cl2/O2 plasmas. The absolute concentrations of SiClX (X = 0–2) radicals have been measured by broad band UV absorption spectroscopy at different O2 gas flow rates in the process gas mixture, and at different RF powers injected in the plasma. At the same time, the chemical composition of the layer deposited on the reactor walls has been investigated by x-ray photoelectron spectroscopy analysis. Without O2 in the plasma, the reactor walls stay clean because the silicon containing compounds redeposited on them (from Si, SiCl, Si+ and SiCl+ precursors) are subsequently etched by Cl atoms and recycled back into the plasma as SiCl2 and SiCl4 volatile species. Hence, the reactor walls are a region of production for these species, leading to their high concentrations in the gas phase. The introduction of O2 gas into the plasma results in the oxidation of the silicon chloride radicals resident on the surfaces and in the appearance of a silicon oxychloride layer on the reactor walls, whose deposition rate increases rapidly with the O2 flow. As a consequence, the production rate of SiCl2 by the reactor walls decreases because a part of the silicon containing species redeposited on the reactor walls is oxidized and incorporated in the silicon oxychloride film instead of being recycled back into the plasma as SiCl2 and SiCl4. Finally, a simple zero-dimensional model is built to predict the densities of SiClX radicals from the measured densities of SiClX+1 radicals and ions. The comparison between the calculated and measured densities at different RF powers allowed us to conclude that SiCl and Si radicals are produced both in the gas phase by electron impact dissociation of SiCl2 (SiCl) radicals, and at the reactor walls by the neutralization and reflection of approximately 50% of the SiCl+ (Si+) ions impinging on these surfaces. At the same time, SiCl and Si are estimated to be lost (adsorption and abstraction reactions) on the reactor walls with a probability ranging between 0.2 and 1.