Smog chamber tests were conducted using automobile exhaust gas generated during emission tests with a group of alternatively fueled vehicles. The tests were designed to evaluate the photochemical characteristics of organic emissions from vehicles operating on compressed natural gas, methanol, ethanol, liquefied petroleum gas, and reformulated gasolines. A description of the emission characterization portion of the study is presented in this paper to assist in better understanding the chamber experiments described in a companion paper. The study is part of a larger program in which the U.S. Department of Energy (DOE), in cooperation with the U.S. Environmental Protection Agency (EPA), is examining the effects of alternative fuel usage on emissions and fuel economy.