In this study, the complexation of Eu(III) and Cm(III) with aqueous phosphates was investigated using laser-induced luminescence spectroscopy. Experiments at 25 degrees C and different ionic strengths (0.6-3.1 mol.L-1 NaClO4) established the formation of EuH2PO42+ and CmH2PO42+. From the conditional stability constants, the respective values at infinite dilution as well as the epsilon(Me(H2PO4)(2+);ClO4-) (Me = Eu or Cm) ion interaction coefficients (using the specific ion interaction theory - SIT) were derived. Further experiments (at constant ionic strength of 1.1 mol.L-1) showed that upon increasing the temperature (25-80 degrees C), the formation of both EuH2PO42+ and CmH(2)PO(4)(2+ )was favored. Using the van't Hoff equation, the molal enthalpy Delta H-R(m)degrees and molal entropy Delta S-R(m)degrees of these reactions were derived, corroborating an endothermic and entropy driven complexation process. This work contributes to a better understanding of the coordination chemistry of both trivalent lanthanides and actinides with phosphate ions.