A reactor model is proposed as a means of representing an in situ redox. fluidised bed reactor for the oxidative dehydrogenation of n-butane with a V-Mg-O catalyst. The model describes a reactor where the hydrocarbon is fed at some intermediate point and the catalyst oxidation and reduction occur in different zones within a fluidized bed. This model allows the behaviour of the reactor to be understood and the effect of operating parameters that cannot be determined by experiments to be predicted.