Electron spin resonance (ESR) spectra of VO2+ radical-ions in sub- and supercritical water are observed. Upon increasing the temperature from 20 to 100 degrees C, the fine structure line widths in the ESR spectra of the vanadyl ion are observed to be reduced that is associated with the effective averaging of the g-factor anisotropy and the hyperfine interaction. With further increasing the temperature, the spectrum components of the hyperfine structure are broadened significantly resulting in the unresolved low-intensity line in supercritical water with Delta H-pp similar to 300 G. The data obtained allow behavior peculiarities of the paramagnetic VO2+ ions in sub- and supercritical water including rotational dynamics and spin exchange between the radicals to be elucidated. The registration of the unresolved low-intensity line in supercritical conditions points to an increase in the local ion concentration in the system that can be an initial stage for the formation of vanadium-based particles in supercritical conditions. The study demonstrates that ESR is the powerful tool to investigate properties of sub- and supercritical water in situ. (C) 2011 Elsevier B.V. All rights reserved.