Geologic sources of obsidian are frequently treated in the archaeological literature as single, discrete flows, each being recognized as a different source as long as it can be geographically isolated. However, some recent studies show a tendency toward the correction of this misconception, and toward defining operationally the obsidian source concept on a regional basis. This work is intended to help achieve such definition for a number of obsidian sources in the central highlands of Guatemala. Geochemical characterization of samples from diverse source localities, attained by means of neutron activation analysis, is utilized for this purpose. The information thus obtained has importance to a number of fields of research in archaeology. To the now-routine studies on prehistoric trade routes, other lines of investigation can be added, namely the determination of source-specific obsidian hydration rates for dating purposes, and the reconstruction of structural features in sociocultural systems.