Atmospheric sulfates are currently a topic of considerable interest. This arises primarily from their possible association with adverse health effects,1 and from their undoubted importance in the acid rain problem.2 Knowledge of the sulf ate species present in airborne particulates is essential to a full appreciation of the sources of aerosol sulfate constituents and to an understanding of respiratory health hazards. Charlson and coworkers3 have recently reviewed the current state of knowledge of the chemical nature of tropospheric Sulfur aerosols. It is clear from Charlson’s review that firm evidence exists for the presence of only a very few specific sulfate compounds in the atmosphere, and that much of our current thought is based upon inference and supposition.3