The 3-dimensional microstructure of a porous electrode from a lithium-ion battery has been characterized for the first time. We use X-ray tomography to reconstruct a 43 x 348 x 478 mu m sample volume with voxel dimensions of 480 nm, subsequent division of the reconstructed volumes into sub-volumes of different sizes allow us to determine microstructural parameters as a function of sub-division size. We show that the minimum size for a representative volume element is about 43 x 60 x 60 mu m for volume-specific surface area, but as large as the full sample volume for porosity and tortuosity. (C) 2009 Elsevier B.V. All rights reserved.