The effects of chloromethane (CH3CI) addition on soot inception in methane-air and ethylene-air counter-flow diffusion flames were investigated by varying the concentrations of chloromethane and nitrogen in the fuel stream. Experiments showed a monotonic increase in the critical sooting stretch rate for methane-air flames when methane was replaced by chloromethane, while ethylene and chloromethane flames exhibited a larger sooting tendency than flames under comparable conditions and burning either ethylene or chloromethane alone. For the conditions investigated, the critical sooting stretch rates of methane-chloromethane-nitrogen flames were shown to be primarily a function of the chloromethane loading in the fuel stream. The structure of these flames was modeled using detailed chemistry and transport. Modeling results suggested that the enhancement of soot formation in ethylene-chloromethane flames may be a combined result of increased concentrations Of C-2 species and chlorinated C, radicals (CH2CI and CHCI). A large rate of the reactions among these species may be the first steps in the molecular growth processes, which leads to the inception of soot particles. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.