Nine Mayan obsidian knife blades from two Late Classic (i.e. 700-900 A.D.) sites in the Yucatan Peninsula, Mexico, have been analyzed by inductively coupled plasma emission spectroscopy (ICPES) for the elements Ca, Fe, K, Ba, La, Mn, Sr, Ti, Zn and Zr, Rb was analyzed by atomic absorption spectrophotometry. These data permit unique assignment of a volcanic obsidian source for each artifact. Obsidian for five of the blades originated from the Pachuca and Zinapecuaro-Ucareo localities near present-day Mexico City; obsidian for the other four was derived from El Chayal, a locality near present-day Guatemala City, Guatemala. These results indicate that extensive trade and supply networks existed during Late Classic times. From purely an analytical standpoint, these results demonstrate that ICPES is a potentially valuable tool in geo-archaeological studies.