The majority of contemporary gas delivery modules, on anaesthetic workstations, are equipped with oxygen-nitrous oxide proportioning systems which should prevent the delivery of hypoxic gas mixtures. We investigated five modules, of two different types, using fresh gas flows ranging from 50 mL min-1 to 20 L min-1 with minimally acceptable proportions of oxygen and maximally acceptable proportions of nitrous oxide set at the flow control valves. The oxygen concentrations of the resulting gas mixtures were measured at the fresh gas outlet of the gas delivery modules. All the modules prevented the delivery of hypoxic fresh gas flows both under normal and abnormal working conditions (unequal supply of gas pressures). All systems showed increased O2 concentrations at fresh gas flows below 1 L min-1. The systems can be used with low flow anaesthesia techniques with one exception (Dameca) also with minimal flow techniques.