The complexation of NpO2+ with sulfate is studied in aqueous solution using absorption spectroscopy. The total ligand concentration (Na2SO4), ionic strength (NaClO4), and temperature (T = 20-85 degrees C) is varied, yielding detailed information on the thermodynamics of the complexation reaction. A single complex species (NpO2(SO4)(-)) is identified by peak deconvolution of the absorption spectra. The molar fraction of the complex species increases with increasing temperature. The conditional stability constants log beta'(1)(T) are calculated and extrapolated to zero ionic strength for each temperature condition with the specific ion interaction theory (SIT). The determined log beta'(1)(T) values increase by approximately one logarithmic unit in the studied temperature range. Furthermore, the log beta'(1)(T) values are linearly correlated with the reciprocal temperature. Thus, fitting the data according to the integrated Van't Hoff equation yields the standard reaction enthalpy (Delta H-r(m)0) and entropy (Delta S-r(m)0) of the formation of the NpO2(SO4)(-) complex. The results show an endothermic reaction which is solely driven by the reaction entropy. In addition, the binary SIT ion-ion interaction coefficient epsilon(Na+, NpO2(SO4)(-)) of the complex species is determined. (C) 2017 Elsevier Ltd.