Soot volume fractions of binary mixtures of butane isomers, ethylene-butane isomers, and propanebutane isomers were evaluated experimentally in diffusion flames on both co- and counter-flow burners. Soot volume fractions were measured by two-dimensional line of sight attenuation of a broadband arc lamp generated light in co-flow flames, whereas in counter-flow flames, attenuation of a small radius laser beam was used. Binary mixtures of iso-butane and n-butane did not show any synergistic effects on soot formation. On the other hand, either n-butane or iso-butane addition to ethylene caused a strong synergistic effect in both types of flames that the soot volume fractions were higher than those of the individual mixture components under the same flame conditions. Binary mixtures of propane and butane isomers, however, didnotdisplay any measurable synergisticeffect onsoot formation.These observations were discussed in the light of mechanisms proposed by previous investigators to explain the synergistic effects detected in the flames of binary mixtures. Current results cast doubt on the universality of the dominance of any of the mechanisms previously proposed to explain the synergistic effects observed with some binary hydrocarbon mixtures. Most of the soot formation studies focus on single-hydrocarbon fuels because of the complex nature of soot mechanismsinflames.Ontheotherhand,mixturesofhydrocarbons, especially binary mixtures, are studied, although to a limited extent, to identify the probable soot formation channels associated with potential interactions between pyrolysis products of the hydrocarbons in the mixture. Several of these studies identified a synergistic effect when two pure hydrocarbons are mixed as the fuel. The synergistic effect in soot formation is described as the measured (or computed) increasesofthesootyieldofabinaryhydrocarbonmixtureover the soot concentrations of any of the two components in the mixture individually under similar flame conditions. The synergistic effect is observed if the addition of one single component fuel to another intensifies some production channels for the formation of soot by the interaction of the pyrolysisproductsofthesefuels.Availablefuelmixtureresults in the literature provide some insight into the precursors that might be the culprit for the synergistic effects. However, there exist significant disagreement among researchers about the results of the fuel mixture experiments and the interpretation of these results. 1-3