For diffusion flames, the combination of oxygen enrichment and fuel dilution results in an increase in the stoichiometric mixture fraction, Z(st), and alters the flame structure, i.e., the relationship between the local temperature and the local gas composition. Increasing Z(st) has been shown to result in the reduction or even elimination of soot. In the present work, the effects of variable Z(st) on soot inception are investigated in normal and inverse coflow flames, using ethylene as the fuel. Use of the inverse coflow flame underscores the validity of these concepts, since the convective field in the inverse flame directs particles into the fuel-rich region. Sooting limits based on particle luminosity are measured as a function of Z(st). The sooting limit is obtained by varying the amount of inert gas until soot appears above a predefined height. For each limit flame, the adiabatic flame temperature is calculated and the flame temperature at the half-height is measured. The flame temperature at the sooting limit is found to increase with Z(st) for both normal and inverse flames. The effects of residence time are also investigated, and the sooting limit inception temperature is found to be dependent on fuel stream velocity for both the normal and inverse configurations. A simple model applicable to oxy-fuel combustion is presented which describes how increasing Z(st) results in the reduction and ultimately elimination of soot. This model assumes that soot inception can only occur in a region where critical values for species, temperature, and residence time are met. The soot inception region is shown to be bounded by two isotherms: a low-temperature boundary that is a function of residence time, and a high-temperature boundary that corresponds to the location of a critical local carbon-to-oxygen ratio. The effect of increasing Z(st) is to move the boundaries of the soot inception zone towards each other, until the zone is infinitely thin and thus the sooting limit is reached. By comparing the model to experimental data, a critical local C/O ratio of 0.53 and a sooting limit inception temperature of 1640 K (for a characteristic residence time of 22 ms) were determined for ethylene. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.