Electrostatic probe measurements are reported that identify flame location, displacement speeds of reaction region, and other flame properties within an industrial furnace that is operated with high-temperature preheated air. The electrostatic probe has advantages over other methods when a furnace is operated with high-temperature air. The probe consisted of a fine detection wire and a supporting tube that played a role of the reference electrode. The reaction regions were found to be widely dispersed and weakened as they moved downstream. However, the ion-current signals still included many sharp peaks, perhaps associated with the thin reaction thickness, contrary to the flame structure expected from the high -temperature air combustion. It was also possible to estimate the displacement speeds of reaction region by using the cross-correlation method between two ion current records detected by parallel detection components. The results demonstrate that the electrostatic probe is useful to detect the structure and state of the reaction mode in industrial furnaces even in the presence of high-temperature air combustion. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.