A process for reacting in a fluid bed reactor at least one oxidizable reactant with molecular oxygen in the presence of a catalytically active fluidized bed of solid particles. In the process a molecular oxygen-containing gas having an oxygen concentration greater than that of air is introduced into the fluidized bed while the fluidized bed is maintained in a turbulent regime. The process is suitable for oxidation, ammoxidation and carboxylation processes, including the production of maleic anhydride, acrylonitrile, ethylene, acetic acid and vinyl acetate.