Abstract: A continuous underway measurement of dissolved gases (O₂, CO₂, Ar) in oceanic waters was developed using membrane inlet mass spectrometry (MIMS, HPR-40, HIDEN, UK).After optimizing experimental conditions, including sampling device, sampleing flow rate, sampling equilibrium temperature, we can use this method to determine the O₂/Ar ratio and pCO₂in the surface seawater.Laboratory experiments showed that the instrument yielded high signal stability and precision.The CV of the O₂, Ar, CO₂, O₂/Ar value during 12 h continuous measurements was 1.57%, 3.75%, 2.21%, 2.61% for a same standard solution respectively.Seven standard calibration curves drawn within 10 days also showed high reproducibility with a relative standard deviation (RSD) of 4.18% for the seven slopes of standard calibration curves.The method was employed to determine O₂/Ar ratio and pCO₂ in the north slope of the South China Sea(19.8°N~20.8°N, 114.7°E~115°E).O₂/Ar ranged from-2.52% to 3.34% with a mean of 0.56%±1.02%, and the mean of pCO₂ was 361.53±40.46μatm.This method can be used to measure dissolved gases (O₂, CO₂, Ar) in oceanic waters directly, fast and with high spatial and temporal resolution, which provides a new approach to the research of net community production and pCO₂ in marginal seas of China.