The rate constant, k(e_aq⁻ + H₃O⁺), for the reaction of hydronium ions with hydrated electrons in supercritical water (SCW) at 400 °C has been evaluated as a function of water density using Monte Carlo track chemistry simulations of the radiolysis of SCW over the range of 0.15–0.6 g cm⁻³. Results are consistent with recent predictions using the so-called “cage effect” model.