Centerline profiles of gas temperature, C1 to C12 nonfuel hydrocarbon concentrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF), and soot volume fraction are reported for coflowing ethylene nonpremixed and partially premixed flames with primary equivalence ratios ranging from 24 to 3. Concentrations of acetylene and C4 hydrocarbons were lower in nearly all of the partially premixed flames than in the nonpremixed flame, whereas concentrations of methane and C3H4 were larger in all of the partially premixed flames than in the nonpremixed flame. These results indicate thar the primary effect of partial premixing is not to uniformly increase the concentrations of pyrolysis products, but to shift, the pyrolysis mechanism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpremixed flame, probably because the shift in pyrolysis mechanism enhances self-reaction of C3H3 radicals. Increases in soot volume fraction and other aromatics were observed that matched the increases in benzene. Profiles of PAH fluorescence agreed closely with those for specific gas-phase PAH such as naphthalene, and the maximum PAH signals were a good predictor of the eventual maximum soot volume fractions. Concentrations of oxygenated hydrocarbons such as formaldehyde and ketene were dramatically increased in the partially premixed flames; for formaldehyde this trend was confirmed with in situ LIF measurements. (C) 2000 by The Combustion institute.