Longitudinal concentration distributions were measured under various conditions in the case of oxygen absorption in standard and multistage slurry bubble columns. By measuring the time at the maximum of residence time distribution in the multistage slurry bubble column and by using the relationships between gas holdup (epsilon(G) = 0.02 similar to 0.06), volumetric mass transfer coefficient, and the mean superficial gas velocity ((u) over bar(G) = 0.7 similar to 2.0 cm/s) in a standard slurry bubble column, the concentration distributions of dissolved oxygen in the multistage slurry bubble column could be estimated by the back flow model, in which perfect mixing in each stage and back flow between stages were assumed. The concentration distributions were expressed adequately by the back flow model.