The influence of oxygen (O-2) concentration and inert on the sooting and burning behavior of large ethanol droplets under microgravity conditions was investigated through measurements of burning rate, flame temperature, sootshell diameter, and soot volume fraction. The experiments were performed at the NASA Glenn Research Center (GRC) 2.2 s drop tower in Cleveland, OH. Argon (Ar), helium (He), and nitrogen (N-2) were used as the inerts and the O-2 concentration was varied between 21% and 50% mole fraction at 2.4 atm. The unique configuration of spherically symmetric droplet flames enables effective control of sooting over a wide range of residence time of fuel vapor transport, flame temperature, and regimes of sooting to investigate attendant influences on burning behavior of droplets. For all inert cases, soot volume fraction initially increased as a function of the 02 concentration. The highest soot volume fractions were measured for experiments in At environments and the lowest soot volume fractions were measured for the He environments. These differences were attributed to the changes in the residence time for fuel vapor transport and the flame temperature. For the He inert and N2 inert cases, the soot volume fraction began to decrease after reaching a maximum value. The competition between the influence of residence time, rate of pyrolysis reactions, and soot oxidation can lead to this interesting behavior in which the soot volume fraction varies non-monotonically with increase in O-2 concentration. These experiments have developed new understanding of the burning and sooting behaviors of ethanol droplets under various O-2 concentrations and inert substitutions. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.