Steeped barley was germinated in laboratory scale solid-state fermenters at Various oxygen partial pressures. Experiments were performed with sterile living barley and with barley colonized by the natural microflora. On line monitoring of CO2 concentration in the off-gas of the reactors reflected the metabolic activity more accurately than heat production measurement. When colonized by micro-organisms, the respiratory activity of germinating barley increased with the oxygen level in the air. However, CO2 profiles of sterile barley did not alter at different oxygen concentrations in the inlet air of the reactor. It is concluded that the germination phase of malting can be optimized by controlling the oxygen supply to the barley based on CO2 production measurements and taking into account microbial aspects.