The Dionysius region is located near the western edge of Mare Tranquillitatis and is centered on Dionysius crater, which exhibits a well-developed dark ray system. Proposed origins for these dark rays included impact melt deposits and dark primary ejecta. The region also contains extensive deposits of Cayley-type light plains. Clementine multispectral images and a variety of spacecraft photography were utilized to investigate the composition and origin of geologic units in the Dionysius region. The portions of the dark rays for which spectral and chemical data were obtained are composed of mare debris contaminated with minor amounts of highland material. Both five-point spectra and values of the optical maturity (OMAT) parameter indicate that the dark rays are dominated by mare basalts, not glassy impact melts. The high-albedo rays associated with Dionysius exhibit FeO and TiO₂ values that are lower than those of the adjacent dark ray surfaces and OMAT values that indicate that bright ray surfaces are not fully mature. The high-albedo rays are bright largely because of the contrast in albedo between ray material containing highlands-rich ejecta and the adjacent mare-rich surfaces. The mafic debris ejected by Dionysius was derived from a dark, iron-rich unit exposed high on the inner wall of the crater. This layer probably represents a mare deposit that was present at the surface of the preimpact target site. With one possible exception, there is no evidence for buried mare basalts associated with Cayley plains in the region.