Synthesis and in situ surface modification of copper manganese oxide nanoparticles have been carried out under supercritical water conditions. Flow type and batch type reactors were used in this study. Effect of different reactors on the phase formation was studied in detail. The addition of organic modifier into the reaction system has significant influence on the particle size, morphology and the dispersion of nanocrystal. The particle size was greatly reduced and single nanocrystal particles were formed by the in situ surface modification of nanoparticles with organic reagents during the synthesis. The Surface modified particles showed a very high surface area when compared with unmodified particles. The particles are dispersed in a binary solvent system where organic-modified particles are dispersed in the upper organic solvent medium. In contrast, the particles without surface modification are settled down in the water phase. (C) 2007 Elsevier B.V. All rights reserved.