In one method of metabolic flux analysis, simulated mass spectrometry data is fitted to measured mass distributions of metabolites that are isolated from cultures with defined feeding of 13C-labeled substrates. Doing so, simulated mass distributions must be corrected for the presence of naturally occurring isotopes. A method that was recently introduced for this purpose consists of consecutive correction steps for each isotope of each element in the considered compound. Here we show that all isotopes of each individual element must, however, be corrected in one single step. Furthermore, it is shown that the source of information with respect to isotopic compositions of the elements needs to be chosen with care. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 80: 477–479, 2002.