A theory of countercurrent processes is presented which is especially applied to distillation. The product of absorption factors on the different trays is the basic function into which other variables e.g. the concentrations, can be expressed. The equation for the conservation of heat is used to calculate the product of absorption factors and it is applicable to the most general case of distillation, even if chemical reactions occur. An explicit solution is given if the volatilities of the components have a constant ratio in the top section and a (different) constant ratio in the bottom section. The reflux is variable. A graphical representation permits to obtain in one diagram a survey of all possible means by which a desired separation can be obtained. The unspecified components are approximately taken into account in this diagram. A special feature of the theory is that the number of components involved, is only of secundary importance.