Spectrophotometric investigations have been carried out in order to understand the interaction of NpO2+ (8.5 × 10–5 M) with Th4+ under varying conditions of nitric acid (1–6 M HNO3), thorium concentrations (up to 0.86 M), and temperature (293–343 K). These studies show the appearance of the signature peak of NpIV (ca. 964 nm) in addition to those responsible for the NpO2+ species (980 nm) and the NpO2+–Th4+ complex (ca. 994 nm). The intensity of the NpIV peak increases with increasing temperature, while that of the NpV peak decreases. The Th interaction is noticed at higher ThIV concentrations, which explains the reason for neglecting the NpV–NpIV interactions. The variation of the stability constant data with temperature suggests both exothermic (at low acidity) and endothermic nature of interactions. Density functional theory (DFT) calculations have shown that the NpO2+–Th4+ interaction is stronger than that of the NpO2+–UO22+ one, which has been attributed to high positive charge on the Th4+ ions.