An observer for a reverse-flow reactor, where the combustion of lean VOC mixtures takes place, is designed in this work and it is demonstrated to allow for a quick and reliable estimation of the inlet pollutant concentration and of the outlet reactant conversion from some temperature measurements in the reactor, even when the pollutant concentration in the feed or the pollutant itself changes and when the reaction is moving toward extinction. The results may be used for control purposes, thus avoiding expensive hardware sensors and time-consuming on-line measurements. (c) 2005 American Institute of Chemical Engineers.