A system has been developed, based on pressure controlled gas pumping, for accurate measurement of total gas consumption or production rates in the nmol/min range, applicable for on-line monitoring of bioconversions in microbioreactors. The system was validated by carrying out a bioconversion with known stoichiometric relation between gas consumption and substrate conversion, that is, the enzymatic oxidation of glucose to gluconic acid. The reaction was carried out in a stirred microreactor with a working volume of 100 mu L, whereby the oxygen consumption was monitored on-line. Subsequently the system was applied to determine the oxygen transfer capacity of the microbioreactor. The dissolved oxygen concentration was measured with an optical dissolved oxygen sensor, which was integrated near the bottom of the reactor. Different stirrer sizes and geometries were investigated for their effect on the mass transfer of oxygen. A maximal k(L)a of 156 +/- 10 h(-1), allowing a maximal O-2-transfer rate up to 50 mmol O-2/L/h, was reached which is sufficient to grow cells aerobically in (fed-)batch mode at relatively high biomass concentrations. (C) 2008 Elsevier Ltd. All rights reserved.