A rapid automated method for isotopic analysis of 13C and 18O in CO2 has been developed. A variety of gas samples containing CO2 can be swept from serological tubes into a helium carrier flow; impurities are separated on a GC column so that a pure pulse of CO2 in He flows into the mass spectrometer. Isotopic ratio determinations are carried out as the pulse passes through the mass spectrometer, allowing a sample to be measured approximately every 4 min. A double, concentric needle-probe is used to flush the sample from the tube so that 100% sample recovery is achieved, maximizing sensitivity and preventing the possibility of fractionation. The precision of the technique, sigma(m-1), is better than 0.2% (0.0002 atom per cent excess) for 13C and 0.4% (0.83 p.p.m.) for 18O for 10 micromol of CO2 at natural abundance. Samples containing only atmospheric concentrations of CO2 can also be analyzed.