We evaluated three commercial indirect calorimetry devices which are used during artificial ventilation. Commercial butane, which had an RQ of 0.615, consumes 6.40 ml oxygen, and produces 3.94 ml CO2/1 ml, was burned in a gas-tight combustion chamber in conjunction with the ventilation of a lung model. During combustion, the flow rate of butane was measured with a soapfilm flowmeter for the calculation of reference values of oxygen consumption (VO2) and CO2 production (VCO2). To investigate the effect of oxygen concentration on the accuracy of these instruments, measurements were carried out at FIO2 values of 0.3, 0.4, 0.5, and 0.6 with a fixed ventilation mode (tidal volume 500 ml; respiratory rate 16 breath/min, intermittent positive-pressure ventilation). For the Datex Deltratrac Metabolic Monitor, the mean relative errors of measured VO2, VCO2, and RQ were all within 4.0%, 2.9%, and 4.0%, respectively. For the Engstrom Metabolic Computer, the corresponding values were 1.4%, 5.7%, and 6.0%, and for the SensorMedics MMC Horizon, 5.7%, 2.9%, and 5.9%.