A theoretical model is developed to describe the intervalence transitions in the mixed-valence N,N,N,N-'(')-tetrakis(4-methoxyphenyl)-1, 4-benzenediamine system. The excited-state energies and transition dipole moments are evaluated by means of time-dependent density functional theory while the shape of the lowest absorption band is analyzed in the framework of a dynamic vibronic model. Our results are discussed in relation with recent experimental data.