The mathematical model of the contact membrane distillation process has such an unknown parameter as the
membrane permeability coefficient. The coefficient of permeability of the membrane takes into account not only the
transfer of heat with the vapor through the pores of the membrane, but also through the polymer structure. In order to
determine the coefficient of permeability, which is represented by variables along the length of the channel and in time,
methods of the theory of optimal control were applied. An algorithm for identifying variable length and time permeability
coefficient is developed. The application of the initial approximation ensures the convergence of the algorithm in the fourth
iteration of the gradient procedure. The refined mathematical model of the contact membrane distillation process can be
used in control tasks.