Hydrogen diffusion in thin magnesium films in lateral film direction is studied by stepwise electrochemical loading via the change in optical light transmission ('electrochemical hydrogenography') during the dihydride formation. During the first loading step, the dihydride front propagation kinetics allows determining the lateral hydrogen diffusion coefficient in the film, by applying an analytical model and, alternatively, by comparison with finite-element simulations. Subsequent loading steps show a time lag behavior in the dihydride front propagation. Therefore, they can be regarded as permeation experiments. These subsequent loading steps can be explained by hydrogen diffusion along grain boundaries and by taking the Gaussian shape of the hydrogen site energy distribution in the grain boundaries into account. In average, an effective hydrogen diffusion coefficient of 3(-2)(+12).10(-12) m(2)/s was found in lateral film direction. This value is the highest value known for hydrogen diffusion in Mg-dihydride, at room temperature. Possible sources for this high lateral hydrogen diffusivity are discussed including preferential hydrogen diffusion along the Mg/Mg-oxide interface, anisotropy of diffusion coefficients and lowering of diffusion energy barrier upon anisotropic film expansion. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.