Abstract During a recent NOAA JGOFS Equatorial Pacific cruise all four analytical parameters of the carbonate system were measured—pH, total alkalinity (TA), total carbon dioxide (TCO 2), and the fugacity of carbon dioxide ( fCO 2). The measurements made during leg 2 on surface waters have been used to examine the internal consistency of the carbon dioxide system in these waters. The internal consistency of the measurements was examined by using various inputs of the measured parameters (pHTA, pHTCO 2, pH fCO 2, fCO 2TA, fCO 2TCO 2 and TATCO 2) to calculate the components of the CO 2 system. The results indicate that the measurements have an internal consistency of ±0.003–0.006 in pH, ±5–7 μmol kg −1 in TA, ±5–7 μmol kg −1 in TCO 2 and ±6–9 μAtm in fCO 2 if reliable constants are used for the dissociation of carbonic acid in seawater. These results indicate that our present understanding of the thermodynamics of the carbonate system in seawater is close to the present accuracy in measuring the various parameters of the system (±0.002 in pH, ±4 μmol kg −1 in TA, ±2 μmol kg −1 in TCO 2 and ±2 μAtm in fCO 2).