The OxyGuard CO2 Analyzer is a novel meter that can directly measure aqueous CO2 gas pressure using a water-resistant gas-permeable membrane and infra-red absorption cell. The pCO2 is converted to a concentration via a solubility factor determined from the calibration procedure and a thermistor. We undertook to independently validate the precision and utility of this meter. Water flow over the probe membrane was a key determinant of the reaction time. At water velocities of ≥30 cm s−1 the time to 99% span was 6–7 min, while at 0 cm s−1 it was 55–60 min. Temperature and CO2(aq) concentration did not appreciably affect reaction time. The meter had a precision of ±0.5 mg L−1 CO2(aq), and high linearity (correlation 0.99–1.01) above 1 mg L−1 in both freshwater and seawater. The standard meter will not be useful for measuring low concentrations such as atmospheric CO2 levels, but will be useful in situations where accurate pH and carbonate alkalinity determinations are difficult to obtain, such as saline waters and waters of high organic loadings.