The formation of polycyclic aromatic hydrocarbons (PAH) and soot has been investigated in atmospheric-pressure, laminar, ethane/oxygen/argon premixed flames as a function of mixture equivalence ratio. Mole fraction profiles of major products, trace aromatics, substituted aromatics, and PAH were quantified by direct gas chromatography/mass spectrometry (GC/MS). Soot particle diameters, number densities, and volume fractions was determined using static (classical) light scattering. The dependencies of flame species mole fraction profiles on equivalence ratio, using the expression, X-i(max) -A(1)phi(ni) were also determined. The parameter ni, an indication of sensitivity to equivalence ratio, for stable aromatic precursors exhibits the following rank order: c-C5H6 (4.21) > C3H4 (3.09) > b-C4H6 (2.43) > C4H4 (2.20) > C4H2 (2.16) > C2H2 (1.66). For aromatic species, the values of ni were in the following order: phenylacetylene (9.33) > benzene (8.17) > indene (7.97) > toluene (6.39). In comparison, PAH species were extremely sensitive to flame equivalence ratios, with the following n values: benzo[a]pyrene (30.37) > acenaphthylene (15.33) > cyclopenta (cd) pyrene (14.58) > fluoranthene (13.52) > pyrene (12.73) > anthracene (11.18) > phenanthrene (10.79) > naphthalene (8.99). (C) 2000 by The Combustion Institute.