Abstract: Thin film of CdS prepared by chemical bath deposition was characterized by XPS and depth profiling. The thin film was etched with Ar⁺ (2×10³eV) for 3 800 s to give 90 layers. Through an ultimate analysis of the 90 spectrampeaks of Cd3d, S2p and O1s in the high resolution XPS spectra and applying depth profiling, the following cognitions were drawn:① besides the surface layer and the glass base liner, contents of oxygen in each of the remaining layer were around 6%, and contents of CdS were attained to 94%; ② in order to obtain an homogeneous and even thin film of CdS, the preparation should be completed in one stroke; ③ during the inducing period for formation of the thin film of CdS, binding of Cd²⁺ with S^2- was the main reaction, and that no formation of CdO and Cd(OH)₂ was found in this period; ④ causes for formation of Cd(OH)₂ were the hydrophilicity of the glass base liner and the dissociation of aqueous ammonia which gives OH^- group for forming Cd(OH)₂; while the presence of CdO was due to the decomposition of Cd(OH)₂ during evaporation. ⑤ it was found by semi-quantification that the relative contents of Cd(OH)₂, CdO and adsorbed oxygen were 2.7%, 2.2% and 1.1% respectively as in respect to the content of CdS of 94%.