A numerical study of natural convection heat and mass transfer with film evaporation and condensation in vertical concentric annular ducts is performed. The emphasis is focused on the effects of the film evaporation and condensation along the wetted walls on the heat and mass transfer in vertical annuli. The numerical results, including the velocity, temperature and concentration profiles, local Nusselt and Sherwood numbers and induced volume flow rate are presented for air-water vapor system under different wall temperature T-1 and ratio of radii lambda. The results show that tremendous enhancement in heat transfer due to the exchange of latent heat in association with film evaporation and condensation was found. In addition, the extent of augmentation of heat transfer due to mass transfer is more significant for a system with a higher wetted wall temperature T-1. (C) 2001 Elsevier Science Ltd. All rights reserved.