Abstract A program of aerosol collection was conducted with the Goetz Aerosol Spectrometer in the Los Angeles and San Francisco Bay areas during the spring and summer of 1964 and 1965. These collections yielded more than seventy individual size distributions of sulfurcompound particulates. Weighted averages, calculated from the individual collections, are presented for data classified according to humidity, time of day, and location. A typical size distribution for all the data would have a mass median equivalent diameter of about 0.35 μ with upper and lower quartiles at about 1.0 and 0.1μ, respectively. Sulfur-compound aerosols collected during periods of high humidity had a substantially higher average mass median diameter than those collected under low humidity conditions. A limited number of round-the-clock samples indicated that the average size of the particles generally was largest in the forenoon, decreased in the afternoon and evening, and then increased again during the early morning. There was some variability among the average size distributions for different sampling locations.