Abstract: The volumetric gas-liquid mass transfer coefficients regarding gas absorption in liquid accompanied by instantaneous irreversible reactions for NaOH-SO_2-air system in trickle beds with small particles were determined. Experimental results show that the mass transfer rate in trickle-flow region and pulse-flow region is closely related to the flow patterns of liquids. The correlations of k_Ga with gas flow rate, liquid flow rate, particle size and physical properties of the two fluids are obtained. The experimental conditions were kept as same as those employed in commercial practice.