The Sn[BPO4](0.4) composite material is composed of three main constituents: the electrochemically active Sn-0 species, the BPO4 buffer matrix, and an interfacial amorphous Sn-II borophosphate phase which acts as a link between the other two, improving the cohesion of the whole composite. In this paper, we report an investigation of the effect of the reaction time on structure and size of this interfacial layer formed between the Sn-0 and the BPO4 particles. Sn-119 Mossbauer spectroscopy shows an increase in the amount of the oxidized Sn-II species with the increase of the reaction time following a quasi-kinetic profile. Operando Mossbauer spectroscopy allowed the study of the transformation of the interface at the beginning of the first electrochemical cycle, indicating that the big irreversible loss during the first discharge is closely related to the increase of the amount of Sn-II in the amorphous interface. (C) 2011 Elsevier B.V. All rights reserved.