Seventy-four mass spectrometric analyses have been made on eight isotopic standards for carbon and oxygen. The standards reported on are the National Bureau of Standards limestone, water, and graphite reference samples, the Solenhofen standard used by Niee for absolute measurements, and the carbonate standards used by the Stockholm, Basel, Wellington and Chicago laboratories. The basic measurements are reported as δ values relative to the Chicago PDB standard and as absolute isotope ratios. The absolute ratios are based on new figures for Nier's standard, derived from a reevaluation of his data and new measurements on atmospheric oxygen.

The correction factors for instrumental effects and for the nature of the mass spectra have been studied. Simple but precise equations for the mass spectra effects have been derived, which give the correction factors as functions of the measured differences between sample and standard. The oxygen isotopic composition of carbon dioxide produced by combustion of carbon has been studied and the derived correction factors for carbon measurements were checked experimentally by varying the oxygen-18 content of carbon dioxide.