The irreversible H2O-transfer between a drop and moist air is described on the basis of the diffusion model for condensational drop growth. Depending on the chosen form of the flux-gradient relationship to approach the irreversible vapour diffusion flux, distinct equations for the growth rate follow, all of which are equally in accord with thermodynamics. They differ in the formulation of the driving force and in the choice of the relevant moisture variable as well as in the correction term due to thermal effects. Considering all assumptions entering the approaches of the condensational drop growth rate, arguments are provided that the most relevant form of the growth equation should be that depending linearly on the mass fraction of water vapour in air as representative moisture variable. ¿ 1998 American Geophysical Union