Dissolved oxygen/argon (O2/Ar) ratios in the oceanic mixed layer are indicative of net community production (NCP) because O2 and Ar share similar physical solubility properties, but only O2 is biologically produced and consumed. We describe a membrane inlet mass spectrometer (MIMS) that allows continuous high-precision shipboard analysis of O2/Ar ratios and eventually other gases, calibrated with discrete samples analyzed in the laboratory. We also present O2/Ar data from the eastern equatorial Pacific. Short-term reproducibilities of 0.05% were achieved. Meridional gradients and small-scale phenomena were clearly resolved. O2/Ar undersaturations around the equator reflect the interaction of biological and physical forcings. Mixed-layer NCP estimated from wind speed-gas exchange parameterizations was near zero north of 2.75°N, and about 12 mmol m−2 d−1 south of 6.75°S. Ar supersaturations, calculated from MIMS O2/Ar measurements and accompanying O2 concentration measurements, ranged from −0.8 to +3.0%.