Electrochemical activation of heterostructures formed by hydroxide proton and two electronic conductors, with one of them being palladium, initiates formation of protonic heterojunctions PdHx vertical bar BaCe0.9Nd0.1O2.9 - y/2(OH)y and PdHx vertical bar KOH center dot nH2O, where x < 0.02, 0 < y < 0.1, n =0.5 and 1. The equilibrium potential of these heterojunctions relative to the I'd reference electrode is approximately -0.8 V. The temperature interval within which this phenomenon was studied is 360-450 K for barium cerate and 300-373 K and 300-410 K for the hydrated forms of potassium hydroxide with n = 0. 5 and n = 1, respectively. The isotopic effect of the potentials of these heterojunctions with protium replaced by deuterium, both in the electrolyte and in the electrode, was determined as 80 mV for the barium cerate at 425 K, and 105 and 120 mV at 368 K for the potassium hydroxide for n=0.5 and 1.0, accordingly. The deuterated heterojunction was found to have a more negative potential. The reproducibility of the measurements was better than +/- 20 mV A comparison of the experimental values of the isotopic heterojunction potential shift and of the isotopic effect of the zero-point energies of hydrogen atoms dissolved in I'd and of protons involved in the O-H chemical bonding in the electrolyte lattice suggests that quantum effects contribute to the potential of the hydrogen-solution-in-palladium-proton-conductor-with-hydroxide-ions protonic heterojunction. (C) 2006 Elsevier B.V. All rights reserved.