The effect of fuel-side carbon density on the levels of polycyclic aromatic hydrocarbon (PAH) formation in atmospheric pressure, opposed flow, ethane diffusion flames has been studied using heated micro-probe sampling and gas chromatography/mass spectrometry (GC/MS). Mole fraction profiles of about 30 major and minor species, as well as trace aromatic and substituted aromatic hydrocarbons and PAHs (up to C14H10, e.g., cyclopenta[cd]pyrene), were quantified by direct gas analysis of samples withdrawn from within the flame without any pre-concentration. Results indicate that fuel-side carbon density and changes in flame temperature markedly influence the levels of benzene and PAH formed within opposed flow diffusion flames. Significant O-2 penetration was also noted on the fuel-rich side of the flame at a lower fuel dilutions, suggesting PAH formation may be also associated with O-2-catalyzed reactions.