Nanomaterials were introduced in the field of lithium-ion batteries several years ago and have already been implemented into commercial cells recently. While the small size brings new reactivity, the effect of particle size on the electrochemical mechanisms still remains to be clearly understood. In this work, we chose to evaluate the evolution of the reaction mechanism for the LixRuO2 system. We show that size plays the main role on the miscibility gap and the Li non-stoichiometry while, the lithium insertion process always proceeds through a triphasic transition, regardless the crystallite size of the sample studied. Furthermore, in correlation with other studies, we underline the importance of the cell parameters mismatch on the appearance of a buffer phase and the sequence of phase transitions. (c) 2010 Elsevier B.V. All rights reserved.