Reversible results for the adsorption of benzene, toluene and n-heptane vapours on mercury have been obtained. The films were found to be gaseous and obeyed the Volmer equation F(A-b) = kT, where F = spreading pressure, A = area per molecule and b = co-area. The possibility that the films might be immobile was considered and the Langmuir equation was applied but found unsatisfactory. A standard state for the surface phase was defined and the free energy, total energy and entropy of adsorption evaluated. The heat of adsorption was shown to increase with the amount on the surface. A number of phase changes were found to occur after the completion of monolayer adsorption, the most striking being interpreted as the change over from 'flat' to 'vertical' adsorption of the toluene molecules. Others were thought to be either two-dimensional condensation or adsorption of a second layer.