In order to apply penetration theory with a Higbie age-distribution function to the analysis of volatile species transport the concept of an equivalent spherical bubble is proposed. It has the same interfacial area per unit volume and the same void fraction as the actual two-phase mixture. The area-averaged mass transport coefficient is evaluated by the Higbie age-distribution function with the equivalent bubble diameter. Experiments are performed to validate the proposed concept, and the effects of errors of parameters on the volatile species transport model are studied. (C) 1997 Elsevier Science Ltd.