A method for monitoring consumption or release of a gaseous analyte such as oxygen by a liquid sample under investigation includes providing a cuvette (1) having an elongate narrow tube (12) of a material which is substantially gas impermeable and which is at least partly transparent to measurement excitation radiation and emission radiation along some of the length of the tube. The tube (12) has a cross-sectional area of under 1 mm2. The sample (15) is loaded into the cuvette (1), the sample being in contact with a probe in the tube (12), the probe being sensitive to the gaseous analyte, and the liquid having at least one surface and an associated headspace (16). The cuvette, the sample, and the probe are equilibrated at a target measurement temperature. Excitation radiation is directed at a sampling zone of the tube (12) and which is distal from the headspace (16), while maintaining the cuvette at the measurement temperature. The emitted radiation is measured and analysed to determine consumption or release by the sample of the gaseous analyte.