Formate dehydrogenase is an important enzyme for NADH-regeneration in enzyme-catalysed reductions. Methods to determine the activity of this biocatalyst during reaction in aqueous-organic two-phase systems or after immobilisation were therefore investigated. Determination of gaseous CO2 in the headspace of reaction vessels either by gas chromatography or by pressure sensors was found to be a suitable way for deduction of FDH-activity in either of these reaction systems. In the presence of organic solvents, gas chromatography yielded more precise data than pressure sensors, while pressure measurements offer the opportunity to assay continuously the activity of entrapped FDH throughout the whole course of reaction.