The influence of isochronous heat treatments, applied at different temperatures between 1000 and 2000 • C under the same conditions and schedules, on the elemental composition of iron-containing CaO-SiO 2 -P 2 O 5 glass ceramics was investigated by X-ray-induced photoelectron spectroscopy. Differences in the elemental composition, with the exception of the oxygen content, are observed. Major modifications are obtained for the glass ceramic resulting after the heat treatment carried out at 1000 • C. It contains the highest iron concentration on the investigated surface and has a Ca/P ratio about three times higher than the untreated sample.