The optical absorption on synthesized Mn-doped Cu2GeSe3 samples was made to determine the variation of the energy gap Eg as a function of temperature in the range 9 to 300 K at photon energies hν = 0.81 to 0.50 eV. The data are analyzed by considering the Manoogian-Woolley equation and using the Varshni relation for the temperature dependence of semiconductor band gaps. The formula is shown to be compatible with the assumption about the influence of phonons on the band gap energy. Above about 100 K the temperature coefficient of the fundamental edge is dEg/dT = —1.5 × 10—4 eV K. The estimated value of the Debye temperature is in agreement with θD = 170 K reported from specific heat measurements. Additionally, a band gap shift due to the incorporation of manganese in the crystal lattice was observed. The results are discussed in regard to some general trends found in the I2–IV–VI3 compounds.