The obsidian hydration dating of prehistoric sites requires that each type of natural glass be identified on the basis of its elemental constituents in order that the appropriate hydration rate constants may be applied. In New Mexico, obsidian occurs in the form of in situ flows and as secondarily deposited detrital material in the sediments of ancestral watercourses. The chemical analysis of 153 geological and archaeological samples from both contexts has resulted in the identification of nine major obsidian sources. The hydration rates for New Mexican obsidians developed by different researchers are compared and evaluated in light of current knowledge on glass-water interaction. It is argued that a hydration rate developed in silica saturated solution or at 100% relative humidity are the preferred conditions for laboratory hydration rate development.