UV photolysis of 1,4-diaminobenzene isolated in a low-temperature argon matrix has been investigated by Fourier transform infrared spectroscopy with the aid of the density-functional-theory calculation. Infrared bands of an intermediate produced from 1,4-diaminobenzene upon UV irradiation (lambda < 350 nm) are assigned to a semiquinone-type radical, 4-aminoanilino radical. A final product produced from the 4-aminoanilino radical upon shorter-wavelength irradiation (lambda < 310 nm) is assigned as 2,5-cyclohexadiene-1,4-diimine. Optimized structures of 1,4-diaminobenzene, 4-aminoanilino radical, and 2,5-cyclohexadiene-1,4-diimine are compared with one another, resulting in changes of the pi-conjugated system similar to those of the hydroquinone and 1,4-benzoquinone system. In addition, trans-cis isomerization of 2,5-cyclohexadiene-1,4-diimine upon UV irradiation is observed in photoequilibrium, where the cis/trans population ratio depends on the irradiation wavelength.