Abstract Technical modification of the conventional method for the δ 13 C and δ 18 O analysis of 10–30 µg carbonate samples is described. The CO 2 extraction is carried out in vacuum using 105% phosphoric acid at 95°C, and the isotopic composition of CO 2 is measured in a helium flow by gas chromatography/isotope ratio mass spectrometry (GC/IRMS). The feed‐motion of samples to the reaction vessel provides sequential dropping of only the samples (without the sample holder) into the acid, preventing the contamination of acid and allowing us to use the same acid to carry out very large numbers of analyses. The high accuracy and high reproducibility of the δ 13 C and δ 18 O analyses were demonstrated by measurements of international standards and comparison of results obtained by our method and by the conventional method. Our method allows us to analyze 10 µg of the carbonate with a standard deviation of ±0.05‰ for δ 13 C and δ 18 O. The method has been used successfully for the analyses of the oxygen and carbon isotopic composition of the planktonic and benthic foraminifera in detailed palaeotemperature reconstructions of the Okhotsk Sea. Copyright © 2009 John Wiley & Sons, Ltd.