INTRODUCTION The American Petroleum Institute (API) asked SRI to evaluate the effect of motor fuel composition on auto exhaust composition and on the development of photochemical smog manifestations. In particular@ the effect of motor fuel composition on eye irritation@ aerosol formation or visibility reduction@ and oxidant formation were to be investigated. Environmental chambers of various designs have been used successfully to evaluate the smog-forming potential of different materials. Comparison of hydrocarbon reactivity data from different environmental chambers has shown that the results are consistent as far as relative ranking is concerned. Good comparison of absolute reactivities has not been observed@ and the validity of extrapolating chamber data to the real environment has not been demonstrated. These differences may@ in part@ be due to differences in absolute intensity and spectral distribution among the various chambers. However@ it is reasonable to expect that the results of chamber studies on vehicle exhaust should provide a means of rating fuels with respect to their relative smog-forming potential@ since chambers at different locations show little disagreement in relative ranking of hydrocarbon reactivity.