In the present work the degree of deterioration of different historical glasses from the Cathedral of Leon (Spain) has been studied. The glass samples were removed from their original position during the restoration carried out in 19th. Century. Chemical analyses of the bulk glasses, as well as different areas on their surfaces have been characterised by several techniques (optical microscopy, scanning electron microscopy and X-ray diffraction). This study on the degradation of the original glass samples was essential for the knowledge of the successive steps that occur during the whole corrosion phenomenon of glasses conserved outdoors. Likewise, the study allowed the statement of the most adequate criteria for restoration and further protection of such historical glasses. The results obtained were consistent with an intermediate degree of deterioration not described previously in the scientific literature about historical glasses of similar composition. The reaction mechanism involved in the formation of corrosion crusts based on insoluble salts (sulphates) was envisioned as further precipitation from the early formed salts (carbonates). (c) 2006 Springer Science + Business Media, Inc.