The objective of this work is to study the impact of the temperature difference between the streams on the flow dynamics and mixing of supercritical water (SCW) and a model hydrocarbon (n-decane), under fully miscible conditions, in a small-scale cylindrical tee mixer (pipe ID =2.4 mm), at an intermediate inlet Reynolds number of 500 using 3-D CFD simulations. When the water and n-decane streams enter the mixer at inlet temperatures of 800 K and 700 K respectively (Delta T = 100 K), the flow remains laminar and the variations of density and viscosity with temperature do not have a significant impact on the flow and mixing dynamics. However, when the water inlet temperature is 1000 K (Delta T = 300 K), the water-HC shear layer becomes unstable close to x = 5D downstream of the mixing joint followed by shear-layer rollup and transition to turbulence. This leads to significant enhancement in the mixing rate. However, in a simulation of SCW n-decane mixing with the same inlet conditions but with the physical properties held fixed at the inlet values (no variation with temperature), the shear layer remains stable and steady state is reached. It was found that, the large variation of temperature of 300 K within the mixing layer leads to an increase in the local fluid density and a decrease in the local fluid viscosity within the mixing layer attributed mainly to the cooling of water and the heating of n-decane respectively. These physical property variations result in an increase in the local Reynolds number within the shear layer rendering it unstable to perturbations in the flow. Thus, the variations in mixture density and viscosity with temperature under near-critical conditions were found to have a significant impact on the flow and mixing dynamics in the tee mixer. (C) 2014 Elsevier B.V. All rights reserved.