Profitable features of the Na/Tl exchange in the crown-flint glass KF3 are demonstrated for generation of multimode waveguide layers. A remarkable affinity for the large Tl + of the glass KF3, deduced from exchange isotherms under equilibrium conditions, is discussed in connection with the high electronic polarizability of thallium. It is shown in different silicate glasses that in the case of a Na/Tl or K/Tl exchange a sufficiently enlarged content of Tl (or Tl 2O, respectively) decreases the glass viscosity and accelerates the alkali/Tl interdiffusion by itself. This effect causes the well known deviations form Fickian behavior in the alkali/Tl exchange kinetics.