It is established that the phonon energy hν p involved in the formation of Urbach's tail in CuInSe 2 depends linearly on the defect parameter related to its deviation from ideal stoichiometry. The empirical relation E U (T,N,P)=(KΘ/σo)[(l+P)/2+N{exp (Θ/T)-l} -1 ] explains very well the temperature dependence of Urbach's energy E U . A linear correlation is found between hν p and both P and N. The extrapolated value of hν p at P = 0 agrees with the average energy of all the phonon modes and that at N = 0 with T F , the temperature at which structural transition from chalcopyrite to sphalerite phase occurs.