Physical and chemical characteristics of atmospheric carbonaceous aerosol particles are reviewed, and their likely effects on the particulate matter as a reaction medium are discussed. An approach to studying the effects of organic components of atmospheric particulate matter on aerosol organic photochemistry based on this discussion is described. Available information suggests that atmospheric aerosols from common combustion sources, such as wood smoke or diesel soot, are almost entirely carbonaceous, consisting of a relatively thick, possibly liquid organic layer coating an elemental carbon core. This implies that organic compounds are fairly mobile and consequently that overall organic composition of an atmospheric aerosol could play an important role in the photochemical behavior of associated reactive organic substances. In accordance with this a photochemical turntable reactor was used to investigate the effects of organic aerosol constituents dissolved in an organic solvent, using benzanthracene as a model photochemically reactive compound. Preliminary results showed that 4 out of 10 major organic compound classes found in atmospheric aerosols include compounds which accelerated benzanthracene photodegradation. These were methoxyphenols, polycyclic aromatic ketones and quinones, substituted benzaldehydes, and substituted furans. ¿ American Geophysical Union 1996