We applied optical waveguide methods to monitor the metal ion adsorption to ultrathin organic films. As a metal ion-sensitive material we used a polymer with azacrown ether sidegroups which are highly selective to CU2+ cation complexation. Ultrathin films of this material were prepared by applying the Langmuir-Blodgett technique. When exposing such a multilayer structure to an aqueous Cu2+ ion solution, the effective index of the propagating waveguide mode increases, indicating that the ions are complexed to the molecules of the ultrathin film structure. The prepared layered structure was sensitive to solutions with a concentration down to c approximate to 3 X 10(-7) M; the response time was in the range of tau = 20-30 min.