Abstract: A multistage model with liquid phase flowing through a series of equal cells with reflux and gas phase flowing through the same series of cells without reflux has been proposed. The concentration of the reacting gas component dissolved in the liquid at ith cell is assumed to be the mean of those at (i + l)th and (i-1)th cells. The concentrations in liquid and gas, and the gas flow rate along the height of column can be calculated stepwise. This model can be applied also to systems in which reaction may be non-first order, and the flow rate, composition and pressure of gas may be variables. This model has been applied to two small experimental bubble columns and the calculated results of the concentration profiles along the height of columns fit observed data fairly well.