A mathematical formalism is presented that reflects the interrelationship between the gross oxygen isotopic composition of a given carbon dioxide reservoir and its composition in terms of the three most abundant 18O/ 16O-distinctive CO 2 species, i.e., 12C 16O 2, 12C 16O 18O and 12C 18O 2 (representing mass numbers 44, 46 and 48, respectively). Utilizing this formalism, a determination of the relative proportions of the masses 44, 46 and 48 of a CO 2 pool would permit us to calculate the average 18O/ 16O ratio of the CO 2-bound oxygen of such a pool with similar precision as obtained by currently used “conventional” methods of 18O/ 16O analysis. Moreover, if the oxygen component of the CO 2 molecules of such a reservoir were derived from two different sources, the quantitative distribution of the above isotopically different CO 2 variants would allow a determination of the gross oxygen isotopic compositions of the two sources. Application of the functional relationships pertaining to the relative abundances of the three principal 18O/ 16O-distinctive CO 2 molecules also permits quantitative estimates of the efflux of CO 2 from a carbon dioxide pool or, alternatively, the influx into such a pool of CO 2 from an extraneous source. Pending the development of satisfactory mass spectrometric techniques for the determination of mass 48, the proposed approach might prove ultimately useful in refined model parameterizations aimed at a better understanding of environmentally relevant CO 2 fluxes.