Nanosized LiNi0.25Co0.5Mn0.25O2 powders have been synthesized via a reverse-microemulsion process. Monophasic LiNi0.25Co0.5Mn0.25O2 with a R3m structure is successfully synthesized after calcining the dried powders at 800° C for 3 h. A specific model is established for estimating the occupancy of all atoms and the degree of cationic ordering in the R $(3) over bar $m- structural compound. Prolonging the calcination time considerably increases the degree of cationic ordering and the cycleability. The electrochemical characteristics of the LiNi0.25Co0.5Mn0.25O2 powders are markedly influenced by particle size. The discharge capacities of LiNi0.25Co0.5Mn0.25O2 increase significantly with a decrease in the calcined powder size. The improvement in the electrochemical properties of LiNi0.25Co0.5Mn0.25O2 is ascribed to the enhanced crystallinity as well as the nanosize of the microemulsion-derived powders. © 2005 The Electrochemical Society. All rights reserved.