Previously we hypothesized that the reported stability of electrodeposited p-Cu2O films is attributed to the predominant Cu+-terminated (111) surface, where the Cu+-supported O-2 reduction occurs and the H+-assisted Cu2O reduction, which is the decomposition of Cu2O, does not happen. The well-known instability of single-crystal Cu2O is attributed to predominant (211) and (311) surfaces, where the photodecomposition into Cu is inevitable at the exposed O2- sites. To examine our hypothesis, the stability and photochemical behavior were compared between electrodeposited Cu2O films with predominantly Cu+-terminated surfaces and ones with predominantly O2--terminated surfaces. When used as photocathodes, the predominantly Cu+-terminated films did not deteriorate as readily as the predominantly O2--terminated films did. Also, when oxygen was removed from the aqueous solution, the predominantly Cu+-terminated films generated photocurrents one order of magnitude smaller compared to the predominantly O2--terminated films, indicating the stability of the Cu+-terminated surface against photodecomposition into Cu. These results confirmed our hypothesis on the crystal face dependence of p-Cu2O stability as a photocathode. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3089290] All rights reserved.