A reliable and convenient characterization method that provides a detailed hydrocarbon composition profile for transportation fuels is an important part of process optimization directed at reducing regulated emissions. In our previous study (Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Rahimi P. M.; Fairbridge, C.; Malhotra, R.; Coggiola, M. A.; Young, S. E. Energy Fuels 2001, 15 (1), 23-37) we described the development of a modified gas chromatography, field ionization mass spectrometry method for detailed hydrocarbon type characterization of diesel fuel. The method proved to be an invaluable technique for rapid analysis of diesel fuel. It was less time-consuming and more informative than the existing mass spectrometry methods, from a characterization point of view, and it was user-friendly and did not require any major modification to an existing commercial instrument, from an instrumentation point of view. This method correlated well with the other methods for total aromatic and saturate groups, and the data fur aromatic subgroups were verified against the data obtained from other mass spectrometric and nonmass spectrometric techniques. This paper describes a continuing effort to verify the new GC-FIMS method and produce correlations for the saturate;groups, The purpose of this study was to validate GC-FIMS measurements of various saturate types using samples of physically separated fractions, enriched in the individual types by various LC methods described in the literature. In this study, the non-normal paraffinic portion of the saturate fraction of the selected diesel cut was separated into iso- and cycloparaffins. The normal paraffins were quantitatively removed by molecular sieve and additionally determined by high-resolution gas chromatography of the saturate fraction. The contents of iso- and cycloparaffins were calculated gravimetrically in each separated fraction and also analyzed by mass spectrometry methods. The results obtained for the saturate types by different methods were all in good agreement, which demonstrates the applicability of the GC-FIMS method for saturates analysis.