There is an increased interest in carbon dioxide-based cycles as it requires smaller turbines than conventional Rankine cycles operating with steam. The focus of this work is the study of the performance of a Rankine cycle operating between 500 and 700 K (226.85 and 426.85 °C). Optimum mixtures of hydrocarbons and carbon dioxide have been worked out for use as working fluids in the above temperature range. This study shows that the use of multicomponent mixtures results in higher efficiency and smaller systems suitable for solar-powered small-scale power cycles. A thermal efficiency of 26.5% and volumetric work of 1945 J/l have been estimated with optimum multicomponent mixture derived in this work with a heat source temperature of 600 K and operating pressures of 100 and 40 bar at expander inlet and exit, respectively.