Urea was decomposed in sub- and supercritical water in the presence and the absence of hydrogen peroxide at temperatures from 538 to 651 K, 23 MPa and residence times up to 1.5 s in a continuous flow reactor. The initial concentrations of urea varied from 0.005 to 0.5 mol L-1. The major products were carbon dioxide, and ammonia. As a minor product cyanic acid was detected without hydrogen peroxide, and nitric acid and nitrous acid were produced with hydrogen peroxide. The decomposition rates of urea with and without hydrogen peroxide were represented by the first order reaction kinetics. The addition of hydrogen peroxide increased the rates at lower temperatures, but was scarcely effective at higher temperatures. In the absence of hydrogen peroxide, the effects of various additives at concentrations of 0.5 mol L-1 on the decomposition rates at 649 K were examined. The addition of NaOH enhanced them remarkably. The presence of NaCl did not affect the product distribution significantly, but enhanced the rates somewhat, in particular, increasing as approaching to the critical temperature of water. The addition of acids, hydrogen chloride and sulfuric acid, did not increase the rates.