Surface characteristics of several types of fly ash from coal combustors were studied by determining adsorption of nitrogen and water. Specific surface areas determined ranged from 2m 2 g −1 to 40m 2 g −1 demonstrating that fly ash must be considered an “active” solid. Adsorption and desorption isotherms for water vapor and rates of adsorption and desorption of water were determined at 0, 100 and 200°C. All samples adsorbed more than a monolayer of water at 0°C and vapor pressures above 0.4 Torr suggesting that fly ash is covered by a complete monolayer of water under normal ambient atmospheric conditions. Some samples adsorbed measurable quantities of water over the entire temperature range 0–200°C. At 0°C and water vapor pressures greater than 2 Torr, samples demonstrated large increases in weight which could not be explained by multilayer formation. Hysteresis was observed on desorption at 0, 20 and 100°C. Pore size distributions were calculated for four of the fly ash samples. ESCA analysis data are presented for five samples.