This work elucidated the proton-incorporation mechanism in ALD YSZ(1). Isotope (H2O)-H-2 was used as an oxidant to trace proton incorporation. The ratio of ZrO2 to Y2O3 ALD cycles was varied from 1:1 to 5:1. TEM confirmed that the ALD YSZ films grew as fully crystallized columnar grains in the cubic ZrO2 phase. SIMS indicated that the Y3+ and H-2(+) concentrations were linearly correlated, indicating yttria-deposition-induced proton incorporation. XPS confirmed an appreciable amount of Y(OH)(3) proportional to the H-2(+) content in the ALD YSZ, as was also detected by SIMS. Oxide ion vacancies created by the replacement of ZrO2 with relatively small amounts of Y2O3 provided additional vacancies for proton incorporation, resulting in steeper [H-2(+)]/[Y3+] slopes. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.