Decomposition and oxidation of nitromethane, nitroethane, and 1-nitropropane in supercritical Water (SCW) near the critical point have been studied in a flow reactor. It was found that the rate of aliphatic nitro compounds decomposition in SCW decreased with an increase in the number of carbon atoms, while the rate of their oxidation increased with an increase in the carbon number. Experimental data on the decomposition of the studied nitro compounds showed that the apparent rate constants, calculated on the assumption of first-order reaction, increased exponentially with an increase in the pressure at constant temperature. The transition-state theory has been applied to simulate the effect of pressure on the rate constants. Some ideas on the nature of the activation complex have been suggested. The calculated values of the activation volumes in the decomposition reactions of nitromethane, nitroethane, and 1-nitropropane in SCW were similar, which suggested the same nature of the activation complex in all three cases.