The absorption of sulfur dioxide into highly concentrated sodium hydroxide in a spray column from simulated flue gas was investigated. The influences of different operating conditions on the SO2 removal efficiency, such as sodium hydroxide concentration, liquid-to-gas ratio, gas velocity and SO2 concentration, were examined. The overall volume transfer coefficients (k(G)a), under moderate conditions, were obtained by introducing the instantaneous and irreversible chemical reaction into the two-film theory to establish a model that allowed the calculation of theoretical values of the overall volume-transfer coefficient. It was found that the absorption process was controlled by diffusion through gas film when a highly concentrated caustic soda solution was adopted. The overall volume-transfer coefficient was essentially independent of concentration of SO2 in gas phase and positively related to the gas and liquid flow rates. The formula of k(G)a was fitted for the process and found to be in good agreement with experimental results.