Abstract <p>The paper considers a fairly wide class of boundary value problems modeling dynamic heat and mass transfer processes in media containing highly or lowly permeable films. Formulas expressing the solutions of such problems with films via the solutions to the same problems without films are derived. On the basis of general formulas, the following laws were obtained: if a highly permeable film is introduced into processes without films, the potentials at any two points symmetrical relative to the film will change by the same value. Similarly, if a lowly permeable film is introduced into processes without films, the potentials at points symmetrical relative to the film will change by the same absolute value opposite in sign. Expressions for increments of the potentials caused by the presence of films were found. Similar laws with films in the form of a circle were obtained for Poisson’s equation.</p>