LiFe(x)Co((1-x))PO(4) (x=0.2, 0.5, 0.8, 1) solid solutions have been prepared by solid state reaction method to sevatudy the electrochemical performance of these olivine structure compounds as cathode material for lithium-ion batteries. SEM observations, X-ray powder diffraction (XRPD) profiles and Rietveld refinement have been carried out to investigate the structural characteristics and morphology of the samples. During the preparations citric acid was added to the reactants to improve the electronic conductivity of the cathode. The results showed that when present in small amounts, Co exerts a beneficial effect mainly at the first cycles where the additive is not effective. At higher Co levels this positive effect seems to cease, probably is still present but hidden by the progressive electrolyte degradation that causes a decay of the electrochemical performance. (C) 2003 Elsevier Ltd. All rights reserved.