Exploration of the photochemical reactivity of diesel-exhaust hydrocarbons is described. A smog chamber was built, and reactivities of diesel exhaust, gasoline exhaust, and propylene were measured at two chamber temperatures. Experimental data indicated the problems caused by instability of diesel-exhaust hydrocarbons. With reservations due to these problems, the results are interpreted to reflect comparable reactivities for diesel and gasoline exhaust. Work done under an agreement with the Air Pollution Control Office of the Environmental Protection Agency (formerly National Air Pollution Control Administration, U.S. Department of Health, Education, and Welfare).