The performance of vitreous silica seals is governed by the interaction between the molybdenum sealing foil and vitreous silica with respect to stress level and adherence. Mechanisms of sealing failures, i.e., oxidation from the outside and interface corrosion by the filling substance, are discussed with respect to affected lamps and requirements for the seal. With respect to wetting and chemical bonding, particular attention has to be directed toward the physical and chemical nature of second-phase species in the interface, which is discussed for Y2O3 and TiO2. Using MY (Mo-0.47wt% Y2O3-0.08wt% Ce2O3) foils, yttrium-rich clusters with a size of a few nm up to 30 nm can be detected within the vitreous silica at a distance of up to 0.6 mu m from the molybdenum/vitreous silica interface. In the MY sealing foil/vitreous silica interface, yttrium can be detected only locally, but always in combination with silicon. In the case of Mo-0.55 wt% TiO2 foils, titanium can be found at each point measured within a distance of 0.58 mu m from the interface for a vacuum-collapse seal and 0.30 mu m for a pinch seal. Titanium clusters with a size of 3-15 nm can be detected in the case of vacuum-collapse-sealed lamps, whereas in pinch-sealed lamps, only a few clusters up to 5 mu m occur very close to the interface.