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248 nm photosensitivity is studied in thin layers of H2reduced SiO2–GeO2binary glasses deposited onto silica substrates by a flame hydrolysis method. It has been found that several effects take place depending on the time of UV exposure and on the fluence per pulse used. We have called them, `behaviour I, II and III'. The first one corresponds to an increase of volume under irradiation, induced probably by a reoxidation of Ge previously reduced by the pre-irradiation chemical treatment. The second one is a common fusion–ablation process. The third one is still not clear but would imply, a diffusive process from nonirradiated to irradiated regions. These observations contrast with the ones made in nonreduced SiO2–GeO2glass for which densification has been pointed out.