A medium-scale reverse flow reactor for VOC combustion is considered. Heat loss and forced dilution by fresh air at the core of the reactor are accounted for. A countercurrent pseudohomogeneous model with frequency correction is proposed. This model is then used to synthesize an observer. This observer allows an on-line inlet estimate of inlet VOC concentration, and of the temperature profiles in the monoliths. (C) 2004 American Institute of Chemical Engineers.