Single-chamber solid oxide fuel cell is a device where two electrodes of different materials contacting a solid oxide ionic conductor may generate a considerable potential difference and electrical power when supplied by a common fuel + oxidant gas mixture The Au vertical bar YSZ vertical bar Pt system in the CH4 + O-2 gas mixture is one of the simplest examples of such a cell In this article the open-circuit voltage (OCV) of this cell supplied with the gas mixture xO(2) + aCH(4) + (1 - x - a)Ar (where a = 001 0 1 or 0 5) is investigated On the basis of the obtained results as well as those for the xCH(4) + (1 - x)(0 2O(2) + 0 8Ar) (0 <= x <= 1) gas mixture reported in our previous work [Electrochim Acta 50 (2005)2771] we postulate that the OCV of the above system arises as a result of electrode modification resulting from solid carbon deposition in the cell After oxidation of the carbon deposit the system once treated by the gas mixture enabling the formation of the carbon phase shows more and more tendency to generate the OCV The open-circuit potential of the Au electrode depends only on the O-2 concentration in the initial gas mixture whereas in the case of the Pt electrode it becomes dependent on chemical equilibria determining the O-2 content in the converted gas mixture Our results reveal that the OCV achieves a reproducible limiting value of similar to 650 mV which is lower by similar to 400 my than the calculated equilibrium value (C) 2010 Elsevier Ltd All rights reserved