Valle del Azufre is a newly discovered source of high-quality obsidian in central Baja California, which exhibits the most intensive exploitation of any known source in the greater United States Southwest and Northwest Mexico. Over 15 subsurface trenches and at least one adit occur at the source. Evidence for the prehistoric distribution of Valle del Azufre obsidian is presented based on energy-dispersive x-ray fluorescence spectrometry characterization of the source material and artifacts from the Baja California peninsula. Despite the intensity of exploitation at the source, the distribution of Valle del Azufre glass appears to be largely restricted to the central part of the peninsula.