Specific questions regarding the antiquity of major midwestern Hopewell culture sites and their role in regional exchange systems are addressed in this paper through the dating (obsidian hydration) and compositional characterization (neutron activation analysis [NAA] and atomic absorption spectroscopy [AAS]) of obsidian artifacts. The analysis of 34 specimens from the Seip, Mound City, and Hopewell sites, Ohio, and the Naples site, Illinois, increases fivefold the number of chronometric dates available from these sites and expands the sample of compositionally identified specimens beyond those resulting from Griffin et al."s (1969) pioneering work. The resulting hydration dates support earlier estimates of the age of these contexts based on 14C or artifact seriation alone. The range of dates (78 B.C.-A.D. 347) and the compositional variety within the sample favors an expanded view of the nature of obsidian trade in the Midwest to include additional western sources, a longer episode of importation, and possible changes in the sources used through time.