Single crystals of Cu In5 Se8 have been grown by chemical vapor transport. The crystals show a deviation from stoichiometry. The temperature dependence of their optical absorption spectra was investigated in the temperature range of 10-300 K. The variation of the energy gap with temperature was studied by means of a three-parameter thermodynamic model, the Einstein model, and the Pässler model. The values of the band gap at T=0 K, a dimensionless constant related to the electron-phonon coupling, an effective and a cutoff phonon energy have been estimated. It was also found that the major contribution of phonons to the shift of Eg vs T in Cu In5 Se8 is mainly from optical phonons. The presence of Urbach's tail just below the band edge in the absorption spectra of Cu In3 Se5 has been observed. It was shown that the static structural disorders contribute mainly to the absorption below the direct band gap.