The roles of hydrocarbons and nitrogen oxides in smog formation were investigated by the Bureau of Mines. Samples of automobile exhaust with varying hc and nox levels were irradiated in a smog chamber under conditions that resulted in smog levels similar to those observed in the atmosphere. Results were used to define the patterns that describe the dependence of a number of smog manifestations on hc and on nox. Using these dependence data and some simplifying assumptions, estimates were made of the ranges within which hc and nox must lie in order to obtain specified levels of air quality. Work done under an agreement with the National Air Pollution Control Administration, U.S. Department of Health, Education, and Welfare.