In order to study the complicated behavior of technetium observed under various conditions of supercritical water (SCW), i.e. quantitative transfer to solid phase, strong adsorption onto the metal surface, or efficient dissolution from the metal surface, solutions and precipitates prepared in reactors made of Hastelloy C-22 in contact with the SCW containing technetium solutions were studied by absorption spectroscopy, TEM, EDX and electron diffraction. Under SCW with or without H2O2, NiCr2O4 and NiMoO4 was produced as the scales (corrosion products) of Hastelloy C-22. Under SCW with RuO2 the formation of colloidal structure comprised of ruthenium metal and TcO2, which may be named as "Tc-Ru-Colloid", were observed. It is worthy of note that the formation of TcO42- from pertechnatate TcO4- (+0.596 V) and the disproportionation of TcO42- is plausible at present in spite of the moderate reductive atmosphere (-0.533 to +0.114 V) estimated from the observation (the existence of TcO2 and the absence of NiMoO4) by TEM and electron diffraction. (C) 2007 Elsevier B.V. All rights reserved.