The lack of appropriate standards and the similarity of electron impact (EI) ionization mass spectra shown by isomeric alkyl benzenes, indans, and tetralins often makes the identification of specific isomers impossible. Chemical ionization-proton exchange (CIPE) mass spectrometry using methanol-d1 as the reagent gas aids in their identification by determining the number of unsubstituted aromatic positions. After ionization of the aromatic ring (that is, addition of D+), only the aromatic hydrogens undergo exchange for deuterium. The number of deuterium exchanges observed allows one to assign the degree of alkyl substitution on the aromatic ring. The CIPE spectra also provide information on the position of alkyl substitution. For example, ortho and para disubstituted alkylbenzenes show a base peak (100% relative intensity) at the mass representing the exchange of all the aromatic protons compared to an intensity of 30 to 40% for meta-disubstituted benzenes. Gas chromatographic/mass spectrometric analysis of the aromatic fraction of a coal-derived jet fuel using traditional EI ionization showed that this fraction (57% by weight) was composed primarily of alkyl substituted benzenes, indans, and tetralins, but few specific isomers could be identified. The CIPE analysis allowed additional classification of approximately 50 individual alkylbenzene, indan, and tetralin isomers based on the degree of aromatic substitution.