Supercritical water oxidation (SCWO) is considered to be a green technology, providing an effective route to convert waste materials into simpler, less hazardous products. This article reports the use of a physical modelling approach to assess mixing dynamics inside three different types of reactor where supercritical water (water above 374 degrees C and 218 atmospheres) is mixed with a second colder, waste containing, effluent flow. Physical or 'pseudo' modelling was used to simulate the general flow patterns and mixing regimes in transparent pseudo reactors (to allow visualization). Towns water was used to simulate the supercritical water flow (density 998 kg/m(3) and viscosity 1.0 x 10(-3) kg/m s at 25 degrees C and 1 bar respectively) and 40% w/w aqueous sucrose solution to simulate the cold aqueous effluent flow (density 1176 kg/m3, viscosity 6.16 x 10(-3) kg/m s at 25 degrees C and 1 bar respectively). Flow rates of 100's ml min(-1)-1000's ml min(-1) were used to create a range of Reynolds numbers experienced during mixing at supercritical conditions (in laminar and turbulent regimes). Three types of vertical pipe-in-pipe reactor were simulated using this method (counter current and co-current arrangements). This visual technique allowed the quantification of mixing efficiency, as well as identification of issues such as flow recycling, stagnant zones, and other inconsistencies in the mixing dynamics. An upwards co-current arrangement provided the 'best' mixing i.e. with minimal wall contact during the downstream oxidation process. (C) 2016 Elsevier B.V. All rights reserved.