A reliable characterization method giving a detailed hydrocarbon composition profile for transportation fuels is an important part of any process that optimizes fuel reformulation and engine design to reduce regulated emissions. This study describes the development of a modified gas chromatography field ionization mass spectrometry (GC-FIMS) method for detailed hydrocarbon type determination of diesel fuel. Diesel fuels were analyzed by GC-FIMS, and the calculated hydrocarbon type composition profile was compared with that determined by other standard techniques. Results for total saturates, total aromatics, monoaromatics, and polyaromatics contents correlated well. Selected ion chromatograms demonstrated the separation of isoparaffins and normal paraffins in typical diesel fuels. The GC-FIMS method produced results for isoparaffin, normal paraffin, and cycloparaffin contents that did not require sample pretreatment to separate saturates and aromatics fractions. Analysis of a gasoline sample showed excellent agreement between GC-FIMS and detailed hydrocarbon analysis or PIONA for total cycloparaffin content, and reasonable agreement for iso- and normal paraffin contents. The normal paraffin contents of selected diesel blends measured by GC-FIMS correlated well with that measured by other methods. Experiments with internal standardization verified the accuracy of the GC-FIMS method for selected hydrocarbon isomers.