Water content associated with the inorganic fraction of PM2.5 and PM10 mass at San Nicolas Island, Long Beach, Burbank, and Riverside, CA has been estimated by using a thermodynamic gas/aerosol equilibrium model (SCAPE) and the measured aerosol composition data from the 1987 Southern California Air Quality Study (SCAQS). From midnight to the early morning, when the temperature is low and relative humidity is high, water is usually the predominant aerosol substance. Particulate water in the winter is estimated to be considerably larger than in the summer at each of the four sites. The estimated mass of aerosol water at high relative humidities is generally larger than that required to account for the total measured gravimetric aerosol mass, suggesting a loss of water in the measurement of total PM2.5 and PM10 mass. Aerosol acidity was also estimated on the basis of the SCAQS data. Generally pH is estimated to be low for all the sampling sites, with the highest pH values at Riverside (2–4.5) and lowest at Long Beach and San Nicolas Island (-0.8–3.3) in the summer. Wintertime particles are estimated to be less acidic than those in the summer.