Hydrothermal decolorization (color loss) of Nordic fulvic and humic acid solutions at 80-180 degrees C for 0-600 hours was traced by ultraviolet-visible (UV-Vis) spectroscopy. These changes were considered to be due to degradation of humic-like chemical structure and represented by decreases in absorbance at 254 nm (UV254). The temperature dependence of their apparent rate constants obtained by the first order reaction model was well described by the Arrhenius equation giving activation energies of 87.6 and 101 kJ mol(-1) for degradation of fulvic and humic acids, respectively. The degradation rates of humic substances were slower than the formation rates of humic-like substances by the Maillard-type browning reaction, suggesting that humic-like substances can be preserved in the aquatic environments, where their formation (browning) and degradation (decolorization) processes are occurring together. By extrapolation to 15 degrees C of the Arrhenius equation, 99-1980 years of time scales for these processes were estimated. A time scale of changes in UV254 in natural aquatic environments estimated by kinetic analyses on Ago Bay bottom sediments (18 years) were closer to the formation time scales of humic-like substances.