As part of a catalytic study, the bulk structure and surface compositions of the mixed oxides of ceria and lanthana, Ce1-x LaxO2-x/2, have been studied over the entire range of compositions. Between x = 0 and x = 0.6, the system forms solid solutions with the cubic fluorite structure. Beyond x = 0.9 it forms solid solutions with the hexagonal close-packed structure characteristic of lanthana. No phase characterisation was possible between x = 0.6 and x = 0.9. Lanthanum is preferentially segregated at the surface from bulk compositions up to x = 0.9, but further increments of lanthana cause the system to exhibit preferential surface segregation of the cerium moiety. The preparative procedure is shown to establish stationary state surface compositions, enabling classification of the composition data as segregation isotherms. Over the composition ranges of the two solid solutions, that is, x = 0-0.6 and x = 0.9-1, the surface segregation may be quantitatively described by power laws similar to the Freundlich gas adsorption isotherm. (C) 2003 Published by Elsevier Inc.