A plug flow model is presented for the oxidation of hazardous carbon monoxide into inert carbon dioxide taking place in the freeboard region above a freely bubbling fluidized reactor. The extent of the reaction in oxygen- and water vapor-bearing flue gas is described as a function of the fraction of solids ejected into the freeboard from the bed surface, the relative throughput, the gas density, and the global rate of reaction. Experimental measurements were conducted to determine the axial concentration profiles of carbon monoxide and other species throughout the freeboard. Steady-state measurements were carried out in an electrically heated reactor with in inner diameter of 9.36 cm above a gently fluidized bed of heat-resistant ceramsite particles in which a precursive liguid fuel (isopropanol) was burned. The global burnout reaction is assumed to be first order in carbon monoxide and one-half order with respect to oxygen and water vapor. The rate parameters for CO and O-2 reacting in the freeboard were determined from the measured concentration profiles. The numeric solutions of the model equations outline the possibilities and limitations for the performance of an afterburner chamber.