The aim of the present paper is to identify a highly efficient binary mixture to be used in an absorption-demixing heat transformer (ADHT) in order to make it more energy-efficient than the conventional absorption heat transformer (AHT). Firstly, 128 binary mixtures potentially interesting in terms of pure component boiling temperatures and upper critical solution temperature (UCST) were selected among thousands for which experimental information related to partial miscibility was available. Secondly, the parameters of the NRTL activity coefficient model were determined for each selected mixture, showing a good agreement between phase-equilibrium calculation and experimental data points. Hence, with the help of the reliable NRTL model, the performance evaluation of the ADHT cycle operating with various working pairs was carried out through the estimation of the internal temperature lift (Delta T-i). Despite a large number of binary mixtures investigated, only 38 ones showed a positive Delta T-i and the most significant value was about 5 K. Finally, in the light of the undertaken modeling work, it was possible to establish the characteristics of the best binary mixture usable in an ADHT. Consequently, a highly efficient binary mixture exhibiting a Delta T-i value of 50 K for industrial application certainly does not exist, owing to the conflict among its characteristic requirements.