Recent interest in both understanding the patterns of prehistoric exchange systems (Renfrew et al. 1968, Cobean et al. 1971, Earle and Ericson 1977) and establishing accurate obsidian hydration dating rates (Ericson 1975, Kimberlin 1971) have required both the field identification and chemical characterization of obsidian sources within a given region. With these interests in mind, a field survey of obsidian sources in west Mexico was conducted in the summer of 1973 in an area contiguous with that surveyed by Cobean in 1969 (Cobean et al. 1971). Subsequently, the obsidian sources were chemically characterized by instrumental neutron activation (INAA) and statistical analysis. Source-specific hydration rates are suggested by comparing the chemical characteristics of sources with the chemical characteristics of artifacts and identifying the sources of the artifacts. This report presents some basic information and analysis which may be useful for future research in the west Mexican region.