The temperature and ionic strength dependences of the complex formation of NpO2+ with formate in aqueous solution are studied by absorption spectroscopy (I-m = 0.5-4.0 mol kg(-1), T = 20-85 degrees C, [Form(-)](total) = 0-0.65 mol kg(-1)), extended X-ray absorption fine structure spectroscopy (EXAFS) and quantum chemical methods. The complex stoichiometry and the thermodynamic functions of the complexation reactions are determined by peak deconvolution of the absorption spectra and slope analyses. Besides the solvated NpO2+ ion, two NpO2+ formate species (NpO2(Form)(n)(1-n); n = 1, 2) are identified. Application of the law of mass action yields the temperature dependent conditional stability constants log beta(n)'(T) at a given ionic strength. These data are extrapolated to IUPAC reference state conditions (I-m = 0) using the specific ion interaction theory (SIT). The results show, that log beta(0)(1)(20 degrees C) = 0.67 +/- 0.04 decreases by approximately 0.1 logarithmic units with increasing temperature, log beta(0)(2)(20 degrees C) = 0.11 +/- 0.11 increases by about 0.2 logarithmic units. The temperature dependence of the log beta(0)(n)(T) values is modeled with the integrated Van't Hoff equation yielding the standard reaction enthalpy Delta H-r(0) and entropy Delta S-r(0) of the complexation reactions. The results show that the formation of NpO2(Form) is exothermic (Delta H-r(1)0 = -2.8 +/- 0.9 kJ mol(-1)) whereas the formation of NpO2(Form)(2)(-) is endothermic (Delta rH(2)(0) = 6.7 +/- 4.1 kJ mol(-1)). Furthermore, the binary ion-ion interaction coefficients epsilon T(i,k) of the formed complexes are determined in NaClO4 and NaCl media as a function of the temperature. The coordination mode of formate toward the metal ion is investigated by EXAFS spectroscopy and quantum chemical calculations. A coordination of the ligand via only one O atom of formate to the metal ion is identified.