As water approaches supercritical conditions, it undergoes sharp changes in density, dielectrics, surface tension and viscosity. Thus, the mixing of supercritical and sub-critical water streams can be potentially impacted by the vast differences in the properties of each fluid. The reactor configurations employed have been characterised using previously reported experimental techniques for assessing the mixing performance in systems with large density gradients. The results obtained have been used to construct a model framework characterised by the turbulence, relative stream momenta and relative buoyancy of the different streams. This has been applied to rationalise the observed reaction outcomes. The results in this paper demonstrate the need for caution when comparing sets of data from supercritical water reactors, undertaken with differing reactor geometries. (C) 2015 Published by Elsevier B.V.