Abstract: Volumetric mass transfer coefficient kL a was determined by the unsteady state oxygen absorption method for a O2-KCl solution system. Changes of the dissolved oxygen concentration in the bulk liquor with time was monitored through a dissolved oxygen probe. Various effects, such as gas pressure, impeller rotational speed and liquid ratio, on the mass transfer coefficient kL a were investigated. It was found that at high pressures, with the increase of gas phase density, gases tended to form small bubbles in the liquid phase, and as a result a higher rnsss transfer coefficient in the agitated vessel was visualized. When the liquid ratio was 0.5, and the employed tunbins impeller diameter ratio 0.8, about 20% increase, in the value of kLa was obtained when operating under a pressure of 36 atm. instead of atmospheric.