The fate of eight nitrogenous compounds (nitrogen substrates) degraded by supercritical water oxidation in the presence of added methanol was investigated. The effect of methanol addition on the nitrogen removal efficiency was evaluated for each nitrogenous compound. The main (> 99%) inorganic nitrogenous compound observed in the treated water containing acetonitrile was nitrate ion, while in the case of the other substrates it was ammonium ion. Moreover, the nitrogen removal efficiency of urea with acetonitrile increased compared with that of urea with methanol, in spite of the higher initial nitrogen load.