In order to get a thorough comprehension of the mechanisms governing hydrogen insertion into nano-metric metallic films, we have studied ultra-thin Pd/Pt(1 1 1) layers. In this paper we propose an original method allowing the measurement of hydrogen insertion electrochemical isotherms. The use of a hanging meniscus rotating disc electrode and a new calculation approach permit to remove the contributions to the insertion charge of both hydrogen evolution and hydrogen oxidation reactions. Indeed, compared to hydrogen insertion such terms become non-negligible in the case of nanometric deposits, due to their large surface/bulk atom ratio. We have measured hydrogen insertion isotherms for Pd/Pt(1 1 1) films from 14 ML down to 4 ML. Independently from the film thickness, the maximum hydrogen insertion rate (H/Pd)(max) is smaller than that of bulk Pd. The so-called two-phase region is still present, but contrarily to bulk Pd it is characterized by a slope. Both hydrogen solubility and the two-phase domain width diminish with the decrease of the film thickness. In the present work the behaviour of hydrogen electrochemical insertion isotherms is interpreted in the light of the Pd nanofilms structure obtained with in situ surface X-ray diffraction. The lattice constraints induced by the substrate result in a lower insertion rate in the Pd deposit close to the Pt-Pd interface. Only the outermost region of the film is relaxed and behaves like bulk Pd. This description quantitatively accounts for the experimental behaviour of (H/Pd)(max) as a function of the film thickness. The obtained Pd/Pt(1 1 1) films structure also corresponds to the presence of non-equivalent hydrogen insertion sites, surely contributing to the slope observed in the two-phase domain. (C) 2013 Published by Elsevier Ltd.