The influence of the addition of acetylene and polycyclic aromatic hydrocarbons (PAHs) on soot formation was studied experimentally for nonpremixed, axisymmetric, coflowing flames of methane with oxygen and oxygen-enriched air. Four reference flames, containing 21, 35, 50, and 100% of oxygen in the oxidizer stream, were seeded with calibrated levels of acetylene, acenaphthene (C12H10), phenanthrene (C14N10), and pyrene (C16H10). It was found that all additives studied enhance soot formation in the reference flames, with the integral soot content increasing linearly with the additive concentration. The increases in the flames' soot volume fractions induced by the addition of PAH compounds were significantly larger relative to the increase due to acetylene addition. The effect of PAH addition on soot growth became substantially stronger with oxygen enrichment of the oxidizer stream. Conversely the relative increase in soot concentration in the flames seeded with acetylene was nearly the same regardless of oxygen content in the oxidizer stream. Estimated additive-to-soot conversion ratios show high degrees of PAHs transformation to soot for all flames studied. The additive-to-soot conversion ratio for pyrene was found to be nearly 2 and remains practically constant with increase of oxygen content. Comparatively, the additive-to-soot conversion ratio for acetylene was close to 0.3 for methane/air flame and decreased with oxygen enrichment. These results suggest that conversion of PAHs to soot is controlled by a relatively fast kinetic mechanism. The conversion rate of acetylene is much slower and may involve a number of stages, including rate-limiting stage of PAHs formation.