Optical absorption measurements near the absorption edge have been studied in the energy range 3–5.5 eV on pure triglycine suphate (TGS) and TGS crystals doped with Cu2+, Mn2+ or Ni2+ in the temperature range 290–360 K. The temperature dependence of the band gap Eg(T) reveals an anomaly at the phase transition temperature for both pure and doped TGS crystals. Absorption coefficient data in the fundamental region are analysed and interpreted in terms of the allowed indirect transitions. The absorption edge following Urbach's rule exhibited exponential behaviour with temperature. Urbach parameters were calculated at different temperatures and the frequencies of effective phonons and electron–phonon interaction constants were determined for various phases