Some general considerations in the analysis of consumption and release of gas components in reactions of a metal in H-2/H2O/O2 gas are presented. Reactions of iron and copper at 160-300-degrees-C in H-2-O-16/H-2-O-18/O2 gas in the mbar pressure range were studied with mass spectrometry in a virtually closed reaction chamber. Data on oxygen and hydrogen uptake in the reaction products and fraction of oxygen coming from reaction with water lead to conclusions concerning partial and complete water dissociation in the reactions with the metals.