INTRODUCTION Chambers in which systematic studies can be made of the reactions between hydrocarbons and nitrogen oxide in the presence of simulated sunlight have been in use for some twenty years. Such chambers have generally been successful in simulating the gross features of photochemical smog@ such as production of oxidants and eye irritants@ and haze. Inter comparison of the results obtained in these smog chambers has not been extensively attempted until relatively recently (Ref. 1)@ at which time the interest in individual hydrocarbon reactivity measurements led to an understanding of the need to compare the various facilities. Intensive comparisons of results obtained in eleven smog chambers have been performed by the Coordinating Research Council project CAPI-6@ Techniques for Irradiation Chamber Studies@ and CAPA 1-69 (Factors Affecting Reactions in Environmental Chambers). The range of physical characteristics of these chambers is shown in Table 1-1. A group of round-robin tests was conducted using these chambers as follows: (1) irradiation of seven different hydrocarbons with nitrogen oxide; (2) replicate runs to establish reproducibility using the propylene nitrogen oxide system; (3) a reactant concentration study in which 3.0 ppm propylene was reacted with 3@0@ 1.5@ and 0.5 ppm nitrogen oxides (Ref. 2). The differences observed among the chambers could not be accounted for analytically@ and an experimental study of how various design and operational variables affect the photochemical reactions observed in smog chambers was instituted. This is a report of the results of this study@ which are also the first reported results for a smog chamber illuminated by a xenon arc lamp. The facility was developed and initial tests performed under Phase I of the project@ which has been previously reported (Ref. 3). Additional experimental and analytical work is in progress (see Section 5.1) and will be reported upon shortly