Six zeotropic mixtures are proposed for conducting a parametric optimisation of supercritical Rankine cycle powered by low temperature geothermal heat source. Different mixing ratios of two types of zeotropic mixtures were studied and their performance evaluated in the range of process parameters. Optimal operational conditions were identified for each mixture and their advantages over pure fluids quantified. The results indicate that the choice of pressure and temperature at the turbine inlet can be tuned to the mixture properties. Thermal and exergy efficiencies of R-143a/R-124 mixtures were higher than for R-143a/R-C318 mixtures in the range of process parameters studied. Mixture R-143a(0.2)/-R124(0.8) yielded the highest thermal efficiency of 16% at evaporator pressure value of 10 MPa and maximum operational temperature 470 K. The highest exergetic efficiency of 47% was developed by R-143a(0.7)/R-124(0.3) at 3.9 MPa and 365 K. A comparative analysis between the zeotropic mixtures and pure R-143a shows that the cycle efficiency can be improved by 15% at the same operational conditions. (C) 2014 Elsevier Ltd. All rights reserved.