The Ries impact, which today is represented by a 24-km-diameter complex crater, occurred at 15 Ma. It is estimated that a projectile of about 1 km diameter formed a transient crater of 6 to 7 km radius and 2.8 km depth. Impact melt glasses investigated are found within the suevite, the breccia that forms the uppermost layers of the ejecta blanket around the crater. The glasses are considered to represent quenched melts produced as a result of the impact. The oxygen isotope compositions of several glasses sampled from widely spaced localities are very homogeneous with δ 18O values in the range of 6.7 to 7.4‰ and δ 17O of 3.3 to 3.7‰. With increasing devitrification and alteration the δ 18O values increase up to 15.8‰, δ 17O up to 8.1‰. Water content also increases with devitrification from fresh glasses with about 1.3 wt.% to 3.8 wt.% in devitrified glass. δD values decrease with increasing water content from about -87 to -127‰. Whole-rock oxygen isotope compositions of the sedimentary cover sequence range from 17 to 27‰, and crystalline basement rocks range from 8.8 to 13.5‰ for granites and gneisses, whereas amphibolites have δ 18O values of 5.2 and 6.1‰. Models suggesting that the glass in the suevite represents a mixed melt derived from all the rocks present in the suevite in proportion of their occurrence (Engelhardt, 1997) do not agree with the oxygen isotope composition of the glass. The simplest explanation for the homogeneous chemical and oxygen isotope composition is that the glass represents quenched melts from a few closely spaced lithologies only. High Fe and Ni contents in the glass, Cr/Ni and Cr/Co ratios, and relatively low δ 18O values indicate involvement of amphibolites during melting. The balance of the melt was made up of spatially associated granites or gneisses as indicated by the major element and rare-earth element geochemistry. The best agreement between major element and oxygen isotope composition of the glass and model melts is obtained by mixtures of amphibolite and granite in proportions similar to their average occurrence in the fallout suevite.