Highly dispersed nanocrystalline Ru particles were prepared under mild conditions in organic solvents such as xylene (Xyl) or dichlorobenzene (Dcb) from the Ru carbonyl precursor. The route of this chemical synthesis determines the physicochemical properties of these materials. This phenomenon is clearly shown in the voltammetric current-potential characteristics of Ru-x(Xyl) and Ru-x(Dcb). Although both materials have a similar particle size, in the reduced state, Ru-x(Xyl) shows a higher electrochemical pseudocapacitance than Ru,(Dcb) by a factor of 10. However, in ambient conditions, the particles slowly oxidize to an amorphous RuxOy species. Their interfacial behavior is similar. Both species and their differences have been characterized by in situ X-ray diffraction. Samples prepared in xylene, RuxOy(Xyl), can be partly reduced in 1 bar of hydrogen at room temperature to similar to2 nm size hcp ruthenium and grown to similar to8 nm particles at 200 degreesC. In contrast, samples prepared in dichlorobenzene, RuxOy(Dcb), under the same conditions can only be reduced at 140 degreesC, to form perfect 2 nm hcp ruthenium clusters of narrow size distribution. This sample is reversibly reoxidized to the amorphous RuxOy. species at 200 degreesC in oxygen. The large 8 nm Ru particles based on the xylene preparation are, however, stable under these conditions. The driving force for such a difference in the electrochemical behavior lies on the disorder precursor state of nano-ruthenium.