Distillation, as well as the other separation processes absorption and extraction, is governed by mass transfer at the interface between one phase and another. In general this mass transfer is experimentally studied, because of the unknown geometry of the liquid vapour interface. There have been entirely theoretic solutions to the calculation of mass transfer for distillation, assuming that transfer interactions only take place within bubbles. However, the restriction to consider an interface only made up of bubbles is not valid for the entire operation range of distillation trays, because at the upper end of the operation range of a distillation tray a substantial amount of tray holes are jetting instead of bubbling. Therefore this paper presents an enlarged model taking into account three different vapour phases, small bubbling, large bubbling and vapour jets.