The crystal structure of La3+ substituted CeO2 solid solution (LaxCe1-xO2-x/2) has been investigated by X-ray and neutron powder diffractions. The fluorite structure of CeO2 is still maintained up to x=0.4 composition although La3+ is the largest trivalent rare earth ion. For xgreater than or equal to0.5 the crystal structure changes to the pyrochloric cation ordering and the remnant La2O3 is unreacted. This structure can, however, be stabilized by Ti4+ addition or thermal treatment. (C) 2003 Elsevier Ltd. All rights reserved.