The thermodynamic parameters (lnK(Ex), Delta H-o and Delta S-o) for the extractions of americium and europium using CyMe4-BTBP in three various diluents (cyclohexanone, hexanol and nitrobenzene) have been studied. The stoichiometry of the complexes was obtained by slope analysis of the distribution ratios of the metals as a function of the ligand concentration. The influence on temperature on the americium/europium separation was investigated for process development causes. In cyclohexanone it was shown that the stability of the (1:2) complex is stronger in the americium case than in the europium case where the (1:1) seems to dominate. Similarly; the 1: 2 complex are more dominant in the case of americium compared to in the europium complex in hexanol as well. In nitrobenzene both the metals form 1: 1 complexes. The extractions of both the nuclides in the three diluents are exothermic. However, since the europium complexes in hexanol and cyclohexanone were shown to contain both 1:1 and 1:2 complexes no further evaluation of those two systems in particular could be made. The enthalpy for the nitrobenzene system differs radically compared to the two others, having a more prominent decrease in Delta H-o (for example delta H-Am(o) = -44 +/- 6.2 (kJ . mol(-1)) in nitrobenzene compared to delta H-Am(o) = -19 +/- 1.1 (kJ . mol(-1)) in hexanol). In the entropies of the system no differences is seen when comparing the diluents cyclohexanone and hexanol, however, there is a difference in Delta S-o between the two and nitrobenzene, were the change in entropy is negative in the nitrobenzene system and positive in the other two diluents. In addition, it was shown that the temperature affected the separation factor of americium from europium. The separation factors decreases in all the three systems as the temperature increases. This is of importance during process development purposes. (C) 2014 Elsevier Ltd. All rights reserved.