The conditions under which soot is formed vary widely and depend upon several factors, including pressure, temperature, fuel type, combustor geometry, and extent of premixing. Although it is known that partially premixed flames (PPFs) can become either more or less sooting than their nonpremixed or premixed counterparts, the impact of partial premixing on soot formation across a large equivalence ratio and flow range is still inadequately understood. Comprehensive experimental data are relatively sparse for this important configuration. Herein, we report on soot formation in various ethylene/air PPFs utilizing full-field light extinction. The dimensionless extinction coefficient K-ext is an important calibrated constant for the determination of the soot volume fraction for this measurement technique. We find that a value of K-ext = 7.1 provides results that are in good agreement with benchmark literature data for a nonpremixed flame. We examined the soot microstructures for two flames established at phi = infinity (i.e., nonpremixed) and 5. In both cases, the primary particles were found to be nearly spherical. In case of the nonpremixed flame the average primary soot particle diameter was similar to 35 nm, but for the phi = 5 flame it was similar to 20 nm. However, the parameter responsible for the value of Kext is the average aggregate size and not that of the primary particles. The aggregate sizes are similar for the two flames. We consider this as verification of a constant K-ext value over the entire equivalence ratio range. The addition of air to the fuel stream produces an initial increase in the flame height. Further air addition gradually decreases the flame height, which is followed by a more rapid decrease with larger premixing. Likewise, the peak soot concentration first increases with small amounts of air addition (or partial premixing of the fuel stream) and reaches a maximum value at phi similar to 24. With further air addition, as phi decreases below a value of 20, the soot volume fraction considerably decreases. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.