The layered Na3Fe3(PO4)(4) phase prepared by solid-state reaction was studied as positive electrode in lithium batteries. Up to 1.9 Li+ ions/f.u. could be intercalated, and only 1.7 Li+ ions could be extracted between 4.5 and 2 V vs Li+/Li with an average voltage of around similar to 2.8 V. In situ and ex situ X-ray diffraction data and Mossbauer spectroscopy measurements indicate that the intercalation/deintercalation process occurs through a solid solution process and is reversible. To improve its electrochemical performances, Na3Fe3(PO4)(4) was also prepared via a hydrothermal method that leads to a significant particle size reduction. However, no clear improvements of the cycling performances were observed using this material as positive electrode in sodium and lithium batteries. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3442365] All rights reserved.