An experimental system for studying surface reactions in the process of microwave plasma etching has been developed. In the system, a surface etched in the microwave plasma can be analyzed with x‐ray photoemission spectroscopy (XPS) without exposure of the surface to room air. In addition, we have developed a procedure for calculating a thickness of a surface layer stoichiometrically different from the substrate material and densities of atoms in the layer. Chemical changes in etched Si and SiO2surfaces caused by exposing these surfaces to room air are investigated with XPS to show the utility of the system. When the surfaces etched in SF6microwave plasma are exposed to room air, the chemical states of the surfaces change rapidly. This is mainly due to surface oxidation and adsorption of hydrocarbon compounds to the surfaces. The rapid changes are more clearly shown from increases in surface layer thickness and the number of O and C atoms in the layer. It is clarified that exposure of etched surface to room air causes the serious disturbance, and that accurate information can not be obtained any longer. The present system which eliminates this disturbance allows accurate measurement of surfaces for detailed investigation of the surface reaction in microwave plasma etching.