The size dependence of the electrochemical activity of nanocrystalline ruthenium dioxide samples was studied using the lithium insertion reaction. The samples were prepared by a sol-gel reaction followed by annealing of initially amorphous precursor at various temperatures to obtain samples with average particle size ranging between 14 and 25 nm. According to analysis of X-ray diffraction patterns, the RuO2 samples prepared at temperatures below 600 degrees C show internal organization into smaller coherent domains separated by planar defect walls. With increasing annealing temperature the coherent domain sizes grow; this process is accompanied by an increase of the particle internal stress. All prepared samples are active toward lithium insertion. The insertion activity of samples improves with increase of the coherent domain size. The coherent domain size affects the mechanism of the Li insertion process. The samples with large coherent domains show typical sequence tetragonal -> tetragonal expanded -> orthorhombic of phase transitions accompanying the Li insertion process with coexistence of all phases in partially reduced material. The material with small coherent domains shows behavior characterized by a single tetragonal. orthorhombic phase transition. (c) 2005 The Electrochemical Society. All rights reserved.