An experimental method for studying microaerobic fermentations, called oxygen programmed fermentation, is introduced. The method is based on a slow ramp of the inlet oxygen concentration in a chemostat. The mathematical equations governing the dynamics of the system are derived and simulations are made for two principally different cases : a purely respirative organism, and an organism capable of fermentation during oxygen limitation. It is shown that at a suitably chosen ramp rate, the dissolved oxygen concentration in the broth can be made to decrease almost linearly. It is suggested that the greatest use of oxygen programmed fermentation will be in initial experiments. Compared with chemostat studies, a scan of different oxygenation rates will provide a timesaving method of finding the interesting regions for metabolic transitions. Furthermore, it is shown that the method makes it possible to study cell physiology at conditions which would normally lead to washout.