Complexation of neodymium(III) with nitrate was studied at variable temperatures (25, 40, 55, and 70 degrees C) by spectrophotometry and microcalorimetry. The NdNO32+ complex is weak and becomes slightly stronger as the temperature is increased. The enthalpy of complexation at 25 degrees C was determined by microcalorimetry to be small and positive, 1.5 +/- 0.2 kJ.mol(-1), in good agreement with the trend of the stability constant at variable temperatures. Luminescence emission spectra and the lifetime of Eu(III) in nitrate solutions suggest that inner-sphere and bidentate complexes form between trivalent lanthanides (Nd3+ and Eu3+) and nitrate in aqueous solutions. A specific ion interaction approach was used to obtain the stability constants of NdNO32+ at infinite dilution and variable temperatures.