In this paper, an experimental measurement of the diffusion constant of hydrogen in ruthenium is presented. By using a hydrogen indicative Y layer, placed under the Ru layer, the hydrogen flux through Ru was obtained by measuring the optical changes in the Y layer. We use optical transmission measurements to obtain the hydrogenation rate of Y in a temperature range from room temperature to 100 degrees C. We show that the measured hydrogenation rate is limited mainly by the hydrogen diffusion in Ru. These measurements were used to estimate the diffusion coefficient, D, and activation energy of hydrogen diffusion in Ru thin films to be D = 5.9 x 10(-14) m(2)/s.exp (-0.33 eV/k(B)tau), with k(B) the Boltzmann constant and tau the temperature. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.