The fact that hydrogen is formed during the cathodic combustion of moist “detonating gas” (VI) suggests that carbonic oxide can be burnt by steam in the discharge. To test this view, systematic investigations along the following three lines were undertaken:—A study of (i) the effect of hydrogen on the cathodic combustion of carbonic oxide, (ii) the cathodic equilibrium between steam and carbonic oxide, and (iii) the cathodic combustion of mixtures of hydrogen and oxygen in various proportions. In what follows is given an account of the investigation of the effect of dilution with hydrogen on the cathodic combustion of “detonating gas.” The experimental results have shown, inter alia, that hydrogen is, in the absence of sputtered metal atoms, a more powerful promoter of the cathodic combustion of carbonic oxide than steam, and that more carbon dioxide than steam is formed when a mixture of carbonic oxide, hydrogen and oxygen in equal proportions is burnt cathodically. These and the further facts set forth below, considered in conjunction with those previously established in the course of the preceding investigations of this series, have enabled us to put forward in what follows a view of the mechanism of the cathodic combustion of carbonic oxide and of the rôle played therein by hydrogen, steam and metal particles, which is not only in accordance with the said facts, but also appears to us to afford a satisfactory basis for a reasonable explanation of the known facts relating to the combustion of carbonic oxide in flames in general.