Modifications are described which enable a commercially available British mass spectrometer to measure small differences in oxygen and carbon isotope ratios with a precision of ±0.01% and using as little as 0.1 ml. carbon dioxide sample. This precision is needed for geological work but might also find application in the use of stable isotopes of oxygen, nitrogen or carbon as tracers in biological systems, where substantial isotopic enrichment could be detected in samples as small as 0.001 ml. It is shown that a viscous inlet system is not a necessary requirement for precise isotope ratio work.