A liquid chromatographic separation of polycyclic aromatic sulfur heterocycles (PASHs) from the polycyclic aromatic hydrocarbons (PAHs) is recommended to avoid interferences from the PAHs to ensure that the PASH determination is reliable when gas chromatography with mass-selective detection is employed. Examples are included that show that this separation also allows the PAHs to be analyzed correctly without interference of the PASHs. Sulfur aromatic compounds are frequently determined in geochemical investigations of crude Oils, Source rocks, and environmental samples and are used as markers for many properties of petroleum such as indicators for source rocks (carbonate vs siliciclastic) and maturity. Some interferences discussed here have been described previously, but the extent and consequences of them seem not to have found as much attention in practical work as would be expected. The step suggested involves a fast separation on a Pd(II)-containing stationary phase that allows the PASHs to be collected in a separate fraction that can be analyzed without interferences using any gas chromatographic detector. Examples are shown based on an Egyptian crude oil and standard reference material SRM 1580 shale oil.