Results of computations based on a detailed chemical kinetic combustion mechanism and results of experiments are compared to understand the influence of ethanol vapor addition upon soot formation and OH radical concentration in opposed flow ethylene/air diffusion flames. For this work, ethanol vapor was added to either the fuel or the oxidizer gases. Experiment and calculations are in qualitative agreement, and both show differing concentrations of soot, soot precursors, and OH depending on whether the ethanol is added to the fuel or oxidizer gases. An explanation for the observed differences for oxidizer or fuel side ethanol addition to opposed flow ethylene/air diffusion flames is proposed, based on an analysis of the chemical kinetic mechanism used in the computations. (c) 2005 Published by Elsevier Inc. on behalf of The Combustion Institute.