Rutile-type RuO2 powders with different crystallite sizes were obtained by annealing of a nanometric precursor, prepared by a precipitation method, at different temperatures. The effect of crystallite size on the electrochemical Li-intercalation reactivity has been studied. For this type of material the size decrease is detrimental as parasitic reactions become prominent. Furthermore, for samples with crystallite size from 3 to 40 nm, the reaction still proceeds through a triphasic process as underlined by PITT and in situ XRD measurements. (C) 2009 Elsevier B.V. All rights reserved.