[en] Titrations of Pu(IV) with HNO₃ in a series of aqueous HClO₄ solutions ranging in ionic strength from 2 to 19 molal were followed using absorption spectrophotometry. The Pu 5f-5f spectra in the visible and near IR range change with complex formation. At each ionic strength, a series of spectra were obtained by varying nitrate concentration. Each series was deconvoluted into spectra f Pu⁴⁺(aq), Pu(NO₃)³⁺ and Pu(NO₃)₂²⁺ complexes, and simultaneously their formation constants were determined. When corrected for the incomplete dissociation of nitric acid, the ionic strength dependence of each formation constant can be described by two parameters, β⁰ and Δ var-epsilon using the formulae of specific ion interaction theory. The difficulties with extending this analysis to higher nitrate coordination numbers are discussed