Mass transfer coefficient k_ls was measured over a range of flow rates of gas U_g= 0-100 cm.s^-1 and liquid U_l=0.05-25 cm.s^-1 in a column packed with spheres of three different diameters d=2.8-12.7 mm. The systems used were the dissolution of benzoic acid in water and diffusionlimited oxidation of brass with dichromate ion in sulfuric acid solution. The effect of U_g on k_ls is not found at all in gas continuous flow, is the greatest in pulse flow and becomes less significant again in dispersed bubble flow. The value of k_ls increases rapidly around the transition from gas continuous to pulse flow. The enhancement factor β (=k_ls in two-phase flow/k_ls in single-phase flow) increases from 1.2 to 2 with increasing d in gas continuous flow while it equals the reciprocal of liquid holdup in pulse and dispersed bubble flows. A liquid-film analogy in gas continuous flow and a single-phase analogy in pulse and dispersed bubble flows are proposed and the experimental results are examined in the light of them.