A potential problem associated with soot particles in the atmosphere is pointed out, namely, the catalytic activity of such particles for certain atmospheric chemical reactions, including the oxidation of sulfur dioxide to sulfate. Conversion of SO/sub 2/ to sulfate is widely recognized as a major source of acidity in clouds and rain water. A significant component of atmospheric sulfate may be generated by heterogeneous reactions rather than gas phase reactions. Heterogeneous conversion of SO/sub 2/ to sulfate on soot particles was demonstrated in laboratory studies by Novakov and his co-workers in 1974. The range of other atmospherically important reactions that may also be catalyzed by carbon is not known. It seems advisable to consider the catalytic properties of soot when assessing the potential impact of increasing diesel emissions. 8 references.