The supply of oxygen can be improved by the direct addition of hydrogen peroxide to cultures of aerobic microbes expressing suf®cient amounts of catalase. This is of special interest if normal aeration has to be kept low, for instance, in order to minimize evaporation of volatile compounds (either substrates or products) or to minimize foaming. Also, if the mechanical power input to the bioreactor is or has to be limited, addition of hydrogen peroxide may be useful.The appropriate dosage of hydrogen peroxide can be simply determined by a controller of the oxygen partial pressure or of the oxygen content in the exhaust gas using various control algorithms. The added hydrogen peroxide can be either a stabilized concentrate, e.g. 30%, or any dilute form of this. In high density cultures, Pseudomonas cells tolerated even harsh controller disturbances. This approach proved to be very robust and reliable.