A reliable understanding of radiolysis processes in supercritical water (SCW)-cooled reactors is crucial to developing chemistry control strategies that minimize the corrosion and degradation of materials. However, directly measuring the chemistry in reactor cores is difficult due to the extreme conditions of high temperature and pressure and mixed neutron and gamma-radiation fields, which are incompatible with normal chemical instrumentation. Thus, chemical models and computer simulations are an important route of investigation for predicting the detailed radiation chemistry of the coolant in a SCW reactor and the consequences for materials. Surprisingly, information on the fast neutron radiolysis of water at high temperatures is limited, and even more so for fast neutron irradiation of SCW. In this work, Monte Carlo simulations were used to predict the G values for the primary species e^–_aq, H^•, H₂, ^•OH and H₂O₂ formed from the radiolysis of pure, deaerated SCW (H₂O) by 2 MeV monoenergetic neutrons at 400°C as a function of water density in the range of ∼0.15–0.6 g/cm³. The 2 MeV neutron was taken as representative of a fast neutron flux in a reactor. For light water, the moderation of these neutrons after knock-on collisions with water molecules generated mostly recoil protons of 1.264, 0.465, 0.171 and 0.063 MeV. Neglecting oxygen ion recoils and assuming that the most significant contribution to the radiolysis came from these first four recoil protons, the fast neutron yields were estimated as the sum of the G values for these protons after appropriate weightings were applied according to their energy. Calculated yields were compared with available experimental data and with data obtained for low-LET radiation. Most interestingly, the reaction of H^• atoms with water was found to play a critical role in the formation yields of H₂ and ^•OH at 400°C. Recent work has underscored the potential importance of this reaction above 200°C, but its rate constant is still controversial.