This two-part paper investigates the potential of ORC (organic Rankine cycles) for the exploitation of low-medium enthalpy geothermal brines. Part A deals with thermodynamic analysis and optimization, while Part B focuses on economic optimization. In ORC field the wide range of available working fluids and cycle configuration entails a non-univocal selection of fluid and cycle parameters for the exploitation of a given heat source. A Matlab((R)) code was created in order to define the optimal combination of fluid, cycle configuration and cycle parameters. Thermodynamic properties of fluids are taken from Refprop((R)) database. An extensive thermodynamic analysis is performed considering geothermal sources in the temperature range of 120-180 degrees C. All the assumptions for calculating the plant components performance are set on the basis of data from literature and real power plants data sheets. Thermodynamic optimization results, shown in terms of reduced variables, allow defining some general rules for the selection of the optimal combination of working fluid and cycle configuration. In particular, it is found that configurations based on supercritical cycles, employing fluids with a critical temperature slightly lower than the temperature of the geothermal source, lead to the highest efficiencies for most of the investigated cases. (C) 2013 Elsevier Ltd. All rights reserved.