A new methodology has been developed to study the thermochemical features of the biphasic transfer reactions of trisnitrato complexes of lanthanides and americium by a monofunctional solvating ligand (tri-n-octylphosphine oxide, TOPO). Stability constants for successive nitrato complexes (M(NO3)(x)(3-x)(aq) where M is Eu3+, Am3+, or Cm3+) were determined to assist in the calculation of the extraction constant, Kex, for the metal ions under study. Enthalpies of extraction (triangle H-extr) for the lanthanide series (excluding Pm3+) and Am3+ by TOPO have been measured using isothermal titration calorimetry. The observed ?Hextr were found to be constant at similar to 29 kJ mol(-1) across the series from La3+ to Er3+, with a slight decrease observed from Tm3+ to Lu3+. These heats were found to be consistent with enthalpies determined using van't Hoff analysis of temperature dependent extraction studies. A complete set of thermodynamic parameters (triangle G, triangle H, triangle S) was calculated for Eu(NO3)(3), Am(NO3)(3), and Cm(NO3)(3) extraction by TOPO and Am3+ and Cm3+ extraction by bis(2-ethylhexyl) phosphoric acid (HDEHP). A discussion comparing the energetics of these systems is offered. The measured biphasic extraction heats for the transplutonium elements, triangle H-extr, presented in these studies are the first ever direct measurements offered using two-phase calorimetric techniques.