The rate of oxidation of a substrate is often divided by the numerical estimate of a correction factor for the proportion of labeled CO2 that is excreted during the course of the experiment. This factor, derived from tracer studies of bicarbonate kinetics, equals the product of the net rate of respiratory CO2 excretion and the ratio of the plateau isotopic enrichment of CO2 to the rate of infusion of labeled bicarbonate. The inverse of this ratio may be equated to the rate of appearance of unlabeled CO2. When one substitutes the expression for the correction factor into a typical equation for the rate of substrate oxidation, the rate of appearance of CO2 substitutes for both net CO2 excretion and the correction factor in the original oxidation equation. Thus the rate of appearance of CO2 is an index of CO2 production that takes into account both net fixation of CO2 and isotopic exchange of labeled CO2; it also may have application as an index of net CO2 production.