Due to unavoidable formation of metal complexes derived from nitrate solutions in high temperature water, the current research aimed at study on such intermediates in order to find the best condition for the complex-free synthesis of metal oxides in supercritical water (SCW). Here, aqueous solutions of several metal nitrate salts including zinc(II), copper(II), manganese(II), cerium(III), magnesium(II), nickel(II), silver(I), and zirconium(IV) have been treated as the precursors for the metal oxide nanoparticle synthesis in a batchwise SCW reactor. The experiments were conducted at two temperatures with different reaction properties, near-critical condition (400 degrees C) in which water preserves its ionic properties relative to the second point with higher temperature (500 degrees C) that the mechanism of reaction inclines to the free radical mechanism due to the falling of dielectric constant (epsilon). Based on the results of X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectra, and thermogravimetric analysis (TGA), it was revealed that some nitrate, hydroxide, and especially hydoxynitrates complexes with ionic origins through their synthesis path, as the byproducts of metal oxide formation reaction, were formed at low temperatures. However, these complexes decomposed into the metal oxide nanoparticles at higher temperature. Complementary experiments on copper nitrate solution showed that temperature is a key parameter in the formation and decomposition of metal complexes in the SCW treatment of metal nitrate solutions used in this research work. Finally, a general mechanism describing the formation and conversion of metal complexes during the treatment of aforementioned metal nitrate solutions and, consequently, synthesis of metal oxide nanoparticles was suggested. (C) 2015 Elsevier B.V. All rights reserved.