Using high-resolution X-ray analysis, elemental mobility during fibre drawing and the subsequent heat treatment were studied in germanium-doped, erbium-doped and cadmium-sulpho-selenide-doped optical fibres. It was found that all but the monovalent cations were effectively immobile in the silica matrix. There was also some evidence for reduced erbium volatility due to aluminium doping in some fibres.