Light scattering coefficient and particulate sulfur concentrations were monitored continuously in St. Louis from July 1977 to June 1978. The measurements were made with an in situ aerosol thermal analysis system, which performed a thermal separation on each of the two aerosol parameters. Each parameter was divided into a number of components by volatility, so that the corresponding components of light scattering coefficient and particulate sulfur concentration could be compared, both individually and statistically. During one summer event, particulate sulfur compounds (and associated water) were estimated to contribute 60--80% to particle light scattering; during another summer event, the percentage was about 66%; and during one winter event, the percentage contribution was less than 41%. The statistical analyses indicated approximately equal scattering efficiencies for sulfuric acid and ammonium sulfate salt compounds. It is estimated that the average overall sulfur percentage contribution to particle light scattering during the July 1977 to June 1978 study period was 50--75%.