Abstract: XPS studies show that ceric sulfate seems to be a mixed valent compound. The ground state is considered as a mixture of two configurations, i.e. 4f~0 and 4f~1. The products of thermal decomposition of ceric sulfate and hence the valence of cerium are related to temperature. For example, after heating Ce(SO_4)_2·4H_2O at temperature between 250 ℃ and 700 ℃ for 2 hours, cerous sulfate (or probably also cerous oxysulfates) were formed. After heating at 800 ℃ for 2 hours, Ce(SO_4)_2·4H_2O was ultimately decomposed into CeO_2.