To complement the gas chromatography/mass spectrometric separation and detection of 3- and 4-ring organosulfur compounds, established methods for the separation of polyaromatic sulfur heterocycles (PASH) have been coupled to high-performance liquid chromatography/atmospheric pressure chemical ionization/tandem mass spectrometry (HPLC/APCI/MS/MS). The latter technique is capable of detecting both thermally unstable and nonvolatile PASH compounds in a crude oil. Our protocol utilizes distillation, alumina, and ligand exchange chromatography and HPLC to partially fractionate the crude oil. Postcolumn addition of tropylium cations prior to APCI/MS leads to an increase in the sensitivity of the detector toward 3- and 4-ring organosulfur compounds. Nonalkylated species primarily generate radical cations in the A.PCI source, whereas alkylated species exhibit both radical cation and product ions resulting from addition and/or loss of hydrogen and loss of alkyl groups. MS2 and MS3 fragmentation patterns confirm the presence of dibenzothiophene and benzonaphthothiophene derivatives in an Arabian crude oil.