Photoimmunotherapy (PIT) is a novel cancer therapy that utilises tumour-specific monoclonal antibodies coupled with a silicon phthalocyanine derivative with biaxial ligands (IR700). Near-infrared (NIR)-induced cleavage of these axial ligands induces the aggregation of IR700 on the cell membrane, resulting in cell death. Using electron paramagnetic resonance (EPR) spectroscopy and computer spectral simulation, NIR irradiation of the IR700 aqueous solution under hypoxia revealed induced IR700 anion radicals in the presence of an electron donor. Two types of IR700 anion radicals with different anisotropies were identified as the precursors of the two-step NIR-induced cleavage reaction of the axial ligand of IR700. Under aerobic conditions, NIR irradiation of IR700 aqueous solution with and without the electron donor produced 1O2 and O2.−, respectively. These results suggest that NIR-induced IR700 anion radicals are important for inducing the aggregation of IR700 and O2.−.