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An examination of the practice of derivatizing organic compounds such as fatty acids, sterols and amino acids in relation to subsequent analysis via gas chromatographycombustion-isotope ratio mass spectrometry is presented. Fractionation processes, such as kinetic isotope effects, which cause a deviation in the measured stable carbon isotope ratios (6-values) of derivatized compounds from simple mass balance considerations are examined. Particular attention is paid to reactions that proceed by the cleavage of a carbon-containing bond and reactions that are likely to have kinetic isotope effects associated with them, such as acetylation and diazotization. Isotope fractionation processes other than those which are kinetic based are also discussed, as is the additional imprecision of the calculation of the 6values of sample compounds inherent when derivative carbon is added. Failure to take this imprecision into account when comparing &-values could lead to erroneous conclusions with respect to the magnitude of kinetic isotope effects caused by derivative reactions.

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