We report a comparative study on the optical activity of surface and interior Ge–oxygen deficient centers in pressed porous and sol–gel Ge-doped silica, respectively. The experimental approach is based on the temperature dependence of the two photoluminescence bands at 4.2 (singlet–singlet emission, S1! S0) and 3.1 eV (triplet–singlet emission, T1! S0), excited within the absorption band at about 5 eV. Our data show that the phonon assisted intersystem crossing process, linking the two excited electronic states, more effective for surface than for interior centers in the temperature range 5–300 K. For both centers, a distribution of the activation energies of the process is found. Based on the results of quantum chemical calculations of the electronic structure of (HO)2Ge: molecule it is suggested that the electronic de-excitation pathway involves two excited triplet states (S1! T2! T1! S0) and shows structural dependence on the O–Ge–O angle. 2005 Elsevier B.V. All rights reserved.