For an optimum performance of microaerobic cultures a defined oxygen transfer rate (OTR) is required which must be uniformly distributed in the entire cultivation volume at a dissolved oxygen tension approaching zero. With the 2,3-butanediol production by Enterobacter aerogenes as a model system it could be shown that on a molecular level OTR should be kept in such a level that as much reducing equivalent (NADH(2)) as possible is oxidised by O-2, and at the same time the activity of the tricarboxylic acid cycle is kept at a minimum. It could be also verified experimentally and theoretically on the basis of metabolic pathways that the respiratory quotient (RQ) is a useful control parameter for optimum O-2 supply. Unfortunately, these conditions cannot be easily realised in large scale reactors and a deteriorated performance is often observed. Reactor hydrodynamics strongly influence the growth and metabolism of microbes under microaerobic conditions. Compared to stirred tank reactors, tower reactors have a much higher energy efficiency and can achieve the same productivity and yield. This communication reviews and reassesses some of the experimental results obtained with the microaerobic production of 2,3-butanediol, particularly from a view point of reaction engineering. The concept of time constants of bioreaction, mixing and mass transfer is applied to analyse the effects of reactor hydrodynamics. In addition, some of the unsolved problems and research needs for a reliable bioprocessing under microaerobic conditions are briefly addressed.