Corrosion phenomena on medieval window glass of Austrian provenance are described on the basis of chemical analysis and scanning electron microscopy in combination with energy dispersive X-ray microanalysis and secondary ion mass spectrometry (sims). Similar to European medieval window glass produced north of the Alps between 1100 and 1600 A.D., Austrian glass differs profoundly from antique or from later glass with respect to chemical composition and other properties. Beechwood ash instead of soda was used during that period as raw material for glass production. It introduced large amounts of potash and lime into the glass. This is the reason for all difficulties with the conservation of medieval painted windows. The chemical durability of glass is enormously decreased by both components. Corrosion products and surfaces of naturally weathered glass panes of two types of glasses present in Austrian medieval painted windows have been investigated. The leaching process of the glass surface of medieval window glass containing K₂O, CaO, and SiO₂ as main components led to corrosion products such as gypsum (CaSO₄ . 2H₂O) and under certain circumstances also to syngenite (K₂SO₄ . CaSO₄ . H₂O) being identified by x-ray diffraction analysis. Pitting corrosion and microcracks as initial state of the glass weathering could be detected by means of SEM beneath the corrosion layers or on scarcely weathered panes. Contrary to medieval K₂O-CaO-SiO₂ glass, dark green colored glass panes in Austrian painted medieval windows show only minor corrosion phenomena. In addition to potash, lime, and silica, this type of glass also contains lead oxide as a main component. Investigations by SIMS prove a decreased content of potassium and calcium in the glass surface. Hydrogen, sulfur and chlorine are incorporated due to the leaching process. These surface layers may protect the glass from further weathering.