Stable isotope tracer methods are gaining popularity in many fields, including environmental science, agriculture and medicine. This is particularly true in the clinical sciences, where the use of radioisotopes can be precluded on ethical grounds and 13C and 15N tracers are being used more frequently for metabolic studies. Mass spectrometry is the preferred technique for isotope analysis. Sample preparation techniques for classical isotope ratio mass spectrometry are often tedious and can involve the use of complex vacuum apparatus that is quite foreign to the biological chemist. Organic gas chromatography/mass spectrometry (GC/MS) overcomes many sample preparation problems but may lack the performance needed for isotope analysis. This paper describes the analysis of 13C using an automatic elemental gas chromatograph interfaced to an isotope ratio mass spectrometer, a system which was developed initially for the analysis of 15N. Details of the residual gas background, peak broadening due to high carrier gas pressures, mass spectral linearity and response to changing sample peak heights and variations of the measured isotope ratio with sample pressure are described. Sensitivity and performance data (reproducibility ± 0.62% at natural abundance) are also given. The potential of this system for other gas chromatographic applications, automatic data handling and the possible simultaneous analysis of 13C and 15N are also discussed.