Abstract: A process of treating surfaces of silica or silicate glass, especially in preparation for subsequent coating in the manufacture of optical waveguides includes introducing a gaseous medium containing at least one component which forms hydrogen fluoride when sufficiently heated to the surface to be treated, while an etching zone which covers only a portion of the surface is thus heated, so that the hydrogen fluoride etches the surface only at the etching zone. The temperature is so selected that silicon tetrafluoride formed during the etching is oxidized and the resultant silicon dioxide is deposited from the gaseous medium onto the surface outside of the etching zone to form a fused fluorine-doped vitreous layer on the previously etched portion of the surface.