High-precision trace-element analyses for 208 geological samples representing 25 mesoamerican obsidian sources were obtained using instrumental neutron-activation analysis to measure a total of 28 elements per sample. These are the first detailed chemical studies ever published for many of the source areas. Especially intensive analyses were made for six sources in the states of Veracruz and Puebla in Mexico from the region of Pico de Orizaba volcano. In addition, source determinations are provided for 65 artifacts from the Olmec site of San Lorenzo Tenochtitl�n, Veracruz. The investigations presented here constitute an important basis for associating obsidian artifacts with specific sources, thereby making possible the reconstruction of Prehispanic trade system.