Rheophoresis, i.e. thin‐layer electrophoresis in particulate gels with controlled evaporation, combines in one experiment the chromatographic and the electrophoretic characteristics of macromlecules in such a way that they can be determined independently of each other. In this paper theory and experimental corroboration for molecular size determination are presented. We describe the migration velocity of any molecule as the sum of an electrophoretic, an electroosmotic and a rheophoretic component. The latter is the velocity originating from the liquid flow produced by evaporation. It is a function of the position along the gel axis and the cross‐section available to the molecule, but totally independent of the molecular charge. As a result of the position dependence, the rheophoretic component can be isolated from the two others and the available cross‐section determined by means of a simple plot. Thus, the chromatographic partition coefficient is readily calculated and the effective hydrodynamic radius of the molecule obtained.