The kinetics of multilayer adsorption processes in solution onto an adsorbent has been explored theoretically. The basic assumption was the "Langmuirian character" of the adsorption process. The general analytical solution for multilayer coverage was obtained, which may be used to evaluate the time dependence of coverage in those cases, when the interfacial kinetics is relatively slow compared to the rate of diffusive transport. As a limiting case of equilibrium of the adsorption process, the general solution provides the expression for the Well-known isotherm derived by Brunauer, Emmett, and Teller (BET isotherm). Additionally, useful approximate analytical solutions were derived based on the general solution for the very beginning and the very end of the adsorption process. The theoretical results of this work allow estimating the. influence of the finite adsorption rate on the functional time dependence of coverage given a set of suitable constants.