In order to determine isotopic abundance ratios such as 13C/ 12C, 18O/ 16O or 15N/ 14N, a mass spectrometer reads the ratio of the voltages which are generated by the isotopic ion beams, trapped by a double collector system. The effect of the gas inlet pressure on the voltage ratio in the case of nitrogen and carbon dioxide is determined experimentally, using a Varian M86. A knowledge of this pressure dependence provides a method of determining the voltage ratio, which is independent of the inlet pressure. This paper partly reviews the work of Craig and gives a detailed treatment of the measuring procedure, of the corrections to be applied to the voltage ratios, viz. for pressure effects, of the way in which the voltage ratios are transformed into isotopic abundances, and of disturbing factors such as spectrometer background and sample contamination. The corrections will be discussed successively in the order in which they are to be applied. In every case the exact equation is deduced, while often an approximation is given for use with natural samples. The major contributions to the technique of mass-spectrometric analysis of isotopic abundance ratios are due to Craig (1957) and Dansgaard (1961). The original choice of the latter to use absolute differences between a sample and standard, rather than relative, has been abandoned. Craig made in his correction formula approximations applicable to samples deviating not more than a few percent from the standard and based on an accuracy of 0.1‰, while nowadays the precision is often better than that. Consequently, we thought it necessary to review the whole procedure and make revisions, where necessary. Although the stable isotopes of carbon, oxygen and nitrogen in CO 2 and N 2 respectively are specifically dealt with, the general equations also apply to isotopic analyses on other gases, such as argon ( 36Ar/ 40Ar) and SO 2 ( 34S/ 32S).