A method for transferring a thin layer from a lithium-based first substrate includes proton exchange between the first substrate and a first electrolyte, which is an acid, through a free face of the first substrate so as to replace lithium ions of the first substrate by protons, in a proportion between 10% and 80%, over a first depth e1. A reverse proton exchange between the first substrate and a second electrolyte, through the free face is carried out so as to replace substantially all the protons with lithium ions over a second depth e2 smaller than the first depth e1, and so as to leave an intermediate layer between the depths e1 and e2, in which intermediate layer protons incorporated during the proton exchange step remain. The depth e2 defines a thin layer between the free face and the intermediate layer. A heat treatment is carried out under conditions suitable for embrittling the intermediate layer and the thin film is separated from the first substrate at the intermediate layer.