Summary: Oxygen partial pressures in atmospheric-pressure quench furnaces were found to vary by as much as 0·5 in log fo₂ when changes were made in gas flow rate and direction suggesting that careful analysis of the gas mixture supplied to a furnace is not a guarantee of the oxygen pressure that a sample in the furnace will attain. The use of magnesiowüstite compositions, with variable oxygen contents, as indicators of oxygen pressure is described.