An external electron ionization (EI) source has been interfaced to a 7 T Fourier transform ion cyclotron resonance mass spectrometer and tested for volatile complex mixture analysis. A new Sulfinert®-deactivated inlet system provides continuous stable sample flow to the EI source, leading to stable ion signal (±10% deviation) for 2 h from a 200 nL mixture of 15 n-alkylbenzenes. Ultrahigh-mass resolving power, m/Δm50%≈735 000, was obtained for 1-bromo-2-chlorobenzene with accurate isotopic ratio measurement. Base line resolution was observed for two of the closest commonly encountered mass doublets, C3/SH4 (m2-m1=3.4 mDa at m/z=190) and SH313C/C4 (m2-m1=1.1 mDa at m/z=190). Although hydrocarbons dominate the positive-ion 18 eV EI Fourier transform ion cyclotron resonance mass spectrum from diesel fuels, many sulfur-, nitrogen-, and oxygen-containing compounds were readily observed without prior fractionation. By comparing 18 eV EI Fourier transform ion cyclotron resonance mass spectra of unprocessed and processed diesel fuels, we were able to identify which heteroatom-containing species were removed by processing.