Abstract: Air-water equilibration methods have been commonly used in measurement of partial pressure of CO₂ (pCO₂) in seawater. CO₂ dissolved in seawater is equilibrated between the seawater sample and air in the headspace of an air-water equilibrator. After the equilibrium is reached, CO₂ concentration in the headspace air is analyzed with a gas analyzer, and pCO₂ of seawater sample is calculated. The air-water equilibrator is a key component of a pCO₂ measurement system because the equilibrator performance greatly affects the accuracy and reliability of the obtained pCO₂ data. This paper introduced the model of equilibrium in air-water equilibrator, the method for measurement of the time constant of air-water equilibrator and the factors affecting the time constant. The structures and features of four types of air-water equilibrators (i.e., shower type, bubble type, laminary flow type and the hybrid type) were summarized. The newly reported bubble type and laminary flow type equilibrators, which are based on venture jet aerator and falling film, respectively, were presented. The sizes, operating parameters and time constants of different equilibrators were listed and compared. The factors to be considered in the equilibrator design and application were analyzed. This paper provided the technical information of air-water equilibrators for pCO₂ measurement.