The present note deals with the optimization of the combustion of organic materials to CO2in Pyrex and quartz tubes. The influence of temperature and oxidant on the combustion yield was investigated. Different physical forms of CuO and a CuO/MnO2-mixture were used as oxidants. Possible isotopic effects during combustion were tested by gas isotope ratio mass spectrometry (GIRMS) of the CO2. Excellent results were obtained for combustion in Pyrex at 530 °C using CuO rods mixed with 20% MnO2as oxidant. The stable isotope results for CO2prepared with this method were accurate and showed a very good precision of 0.030 %. Applications for which quantitative combustion is a necessity, we suggest the use of CuO powder (< 100 μm) as oxidant and a combustion temperature of 850 °C.