NO and soot formation were investigated both numerically and experimentally in oxygen-enriched counterflow diffusion flames. Two sets of experiments were conducted. In the first set, the soot volume fraction was measured as a function of oxygen content in the oxidizer jet at constant strain rate (20 s(-1)). In the second set of experiments, the soot volume fraction was measured as a function of strain rate variation from 10 to 60 s(-1) and at constant oxygen content on the oxidizer side. A soot model was developed based on a detailed C, gas phase chemistry. The soot and molecular radiation were taken into account. Numerical results were verified against experimental data. The soot volume fraction was predicted with the maximum discrepancy less than 30% for all cases considered. It was found that oxygen variation significantly modified the diffusion flame structure and the flame temperature, resulting in a substantial increase of soot. The temperature increase promotes aromatics production in the fuel pyrolysis zone and changes the relative contributions of the thermal and Fenimore mechanisms into NO formation. As the strain rate increases, the residence time of incipient soot particles in the high temperature zone is reduced and the total amount of soot decreases. High concentration of soot in the flame leads to enhancement of radiant heat exchange: the reduction of temperature due to radiation was found to be between 10 and 50 K. This caused a reduction of peak NO concentrations by 20%-25%. The increase of oxygen content in the oxidizer stream resulted in a reduction of the distance between the plane of the maximum temperature and the stagnation plane. (C) 2001 by The Combustion Institute.