An apparatus, incorporating the principle of the parallel plate vis cometer, is described which enables the swelling of films to be accurately measured in organic solvents at various temperatures and under diverse flow conditions. The theory is discussed and formulae are derived which accurately predict the sensitivity with both liquids and gases except when the flow is turbulent. The scale of the microscope focusing mechanism used is calibrated by means of a standard slip employing a gas flow technique.A non-destructive method for the measurement of film thickness of gellike films is described in which a flow of nitrogen locates the surface of each film at a constant distance away. The manner of swelling measurement is indicated and in addition the use of the apparatus for desorption measurements upon swollen films is discussed.