The photoreaction of 2-amino-5-methylpyridine was investigated by matrix-isolation infrared spectroscopyand DFT calculation. Photoinduced reversible amino (NC−NH2)−imino (NH−CNH) tautomerism wasfound between 2-amino-5-methylpyridine and 5-methyl-2(1H)-pyridinimine; the amino tautomer changes tothe imino tautomer by UV irradiation (340 > λ ≥ 300 nm) and the reverse change occurs by longer-wavelengthlight irradiation (420 > λ ≥ 340 nm). The results of the CASSCF calculation revealed that the amino−iminotautomerism proceeds in vibrational relaxation process from electronic excited state to the ground state. TheIR spectra of 2-amino-5-methylpyridine in the T1 state and 5-methyl-2-pyridinamino radical were also obtainedby UV irradiation (λ ≥ 300 nm).