The free energy of activation for viscous flow is related to an entropy and energy of activation by the same equations as any equilibrium. This, for the theory of viscosity is perfectly general and independent of the mechanism. The rolling over each other of pairs of molecules lying in adjoining layers is the mechanism which appears to be the most probable, and the equations for this bimolecular flow process are developed here. At low pressures the heat of activation for viscous flow is about one‐third the energy of vaporization, but as the pressure is raised, it increases rapidly because of the work term,P V/n′. HerePis the external pressure,Vis molal volume andV/n′is the extra volume required before the flow process can take place. Calculations made forn‐pentane, ether, benzene, iso‐pentane, water, and mercury over as extended a temperature and pressure range as the data permit are found to agree satisfactorily with the experimental viscosity. The results are interpreted in terms of the liquid structure and the mechanism of viscous flow. The results of applying our theory to the liquids for which the necessary data is available show that the effect of pressure on viscosity can be calculateda priori, with thermodynamic data only, with reasonable success.