During our studies of the methanation reaction over Ni/SiO2catalyst we have employed mass spectroscopy to follow the redistribution of isotopes among the reactant and product species. When deuterium was employed in the studies,1a wide variety of isotopic species resulted. Among the species produced are H2O, D2O, and HOD. If accurate sensitivity and fragmentation data are available on the species in question, matrix algebra may be used to transform mass fragment data into concentrations. For example see the text by Roboz.2