Experiments of dissolution of a soluble boro-silica glass were performed at 37 degrees C in a saline solution without and with 50, 75 and 100 ppm of added silica. After reaction silica concentration was measured by AAS and colorimetry and fibers were investigated by SEM-EDS and XPS. The results give informations on the dissolution velocity of the glass and the factors controlling the formation rate of the gel layer and its composition. The dissolution velocity measured at the unaltered glass-gel interface is not significantly decreased by the silica concentration in solution whatever its origin (leaching of the glass fibres or initial addition). Accordingly, gel formation is controlled by the chemical reactions rate at, or near, the boundary of the unaltered core. The composition and the stucture of the gel layer are depending on the silica concentration in solution above a threshold of about 50 ppm. The framework of the gel appears to play the more important role in the dissolution velocity of the glass.