A new method for the estimation of fuel specific parameters is introduced to characterize the burn-up behaviour of solid fuels, biomass and waste materials under fluidized bed conditions. For this purpose a specially designed gaspotentiometric oxygen sensor probe is used to detect the oxygen consumption during the batchwise fuel combustion directly in the combustion zone of a fluidized bed reactor. As a result of the in situ measurement and the fast response time of the sensor, the combustion behaviour of the fuel can be immediately described by its typical burn-out curve and, moreover, by the burn-out time, the effective reaction rate constant and a relative reactivity value as fuel specific parameters.