The mechanism of the phase drift in electro-optic polymer waveguide devices is presented based on buildup of free ion charge at the electrode-clad interfaces. The difference of the electrode-clad interfaces at the top and bottom electrodes is shown to be important in reducing the drift and that doping of the core with macrocyclic polyethers can suppress this phenomenon. Arrhenius curves of long-term phase drifts using Mach–Zehnder modulators show the activation energies for symmetric electrode interfaces, asymmetric electrode interfaces, and macrocyclic polyether doped cores to be 0.30, 0.28, and 0.15 eV, respectively.