Non-isothermal miscible viscous flow in homogeneous porous media is studied through nonlinear simulation of the process and analysis of breakthrough efficiency. The main focus is on the effect of heat exchange between the displacing fluid and the solid matrix on the resulting instability. Higher diffusion rate of heat in the fluids reduces the effect of lagging between the fronts due to heat exchange. Hence, sweep efficiency of the injected solvent is less sensitive to thermal lag coefficient as the diffusion ratio increases. Variation of heat sweep efficiency with thermal lag coefficient and thermal log mobility ratio has been analysed as well. (c) 2012 Canadian Society for Chemical Engineering