This study was conducted by the U.S. Environmental Protection Agency, Region 9, in order to assist in chemical emergency planning and prevention efforts at the US/Mexico border. The cities of Calexico, California, and Mexicali, Baja California, are one of the six Sister City pairs on Region 9s border with Mexico under USEPAs Border XXI Program. The study identifies the nature, quantities and routes of hazardous substances transported in or near Calexico. Imports and exports, including hazardous waste, are taken into account. The study draws upon all major sources of relevant data at all levels of government. This kind of information is valuable to emergency planners and responders, but is not readily available to them. The study brings together all of the available information into one place. USEPA hopes that the study will serve as a useful tool and an ongoing reference document for emergency planning and response purposes.