We present a magnetic study of vanadium oxide nanotubes (VOx-NTs) with polyaniline (PAni). Transmission electron microscopy (TEM) shows the tubular shape and the multi-wail structure of the nanotubes. The static magnetic susceptibility measured at different magnetic fields shows a Curie behavior, while the magnetization versus magnetic field presents a non-linear dependence at low temperatures. Both experiments can be explained by the presence of paramagnetic ions with S = 1/2. Using Electron Paramagnetic Resonance technique, we identified as V4+(3d(1)) the paramagnetic ions present in the nanostructures. All the experimental results can be explained by a fraction between 14% and 18% of V4+ with respect to the total V atoms in the system. (C) 2007 Elsevier B.V. All rights reserved.