The dissolution of bore-silicate glass fibres in physiological saline solution was studied at 37 degrees C either in a non-refilled or in a periodically refilled reactor. Large variations of the weight losses were observed with time and refilling frequencies. The weight losses were found to increase with the refilling frequency. Sections of altered fibres, studied using scanning electron microscopy, show an outer hydrated layer surrounding an unaltered glass core. The residual silicon- and aluminium-rich hydrated layer (Al/Si=0.2, H2O/Al=16-19) was characterized by X-ray photoelectron spectrometry, energy dispersive spectrometry and thermogravimetric analysis. The thickness of the hydrated layer may be theoretically calculated from the degree of reaction progression. Under unsteady state conditions, most of the dissolution occurs at the fresh glass-hydrated layer boundary, through selective processes. The proposed model explains the persistence of the aluminium-rich residue when dissolution proceeds in non-replenished systems.