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The aim of this investigation was to establish a Raman procedure for on-site identification of stained-glass windows and their deterioration level. This study of a representative series of stained-glass pieces dating from the thirteenth to seventeenth, nineteenth and twenty-first centuries made it possible to classify the glasses into four types: Type 1 (rare, Ca-rich Na silicate), Type 2 (rare, Na-rich, Ca silicate), Type 3 (more frequent, KCa silicate) and a variant of Type 3, referred to as 3bis. Ion exchange of the alkali cations was made in boiling sulfuric acid and related to structural and compositional changes, which were analyzed with IR and Raman spectroscopy, optical microscopy and EDS. Measurements of the film thickness showed a great discrepancy between the ion-exchange rates of KCa (Type 3: 60 µm/h) and NaCa (Type 2: 15 µm/h) silicates with that of Type 1 Ca-silicate (0.05 µm/h). IR and Raman spectra provided proof that the nanostructure of the glass was modified, chiefly by the downward (Raman) and upward (IR) shift of the position of the main SiO stretching peak and a decrease in the intensity of the Boson peak, as well as the Raman 580 cm−1 SiO bending peak. The decrease in intensity of the narrow ∼950 cm−1 Raman peak, assigned to (earth)alkali nano crystallites in the glassy network, is correlated with the K/Na ion loss and glass weathering. Consequently Type 3bis samples are in fact glasses, in which the surface is depleted of K/Na ions. Optical micrographs support the macroscopic compositional and structural heterogeneity of the ancient glasses. This work demonstrates the potential of Raman scattering for in situ measurements of the degree of weathering of ancient stained glasses. Copyright © 2006 John Wiley & Sons, Ltd.