Adsorption of water on aerosol particles has been measured using an ultrafine tandem differential mobility analyzer. The measurements were made with NaCl and (NH4)(2)SO4 particles, between 8 and 15 nm in mobility diameter, at room temperature. The adsorption was found to be stronger on larger particles, indicating that the Kelvin effect plays a significant role in adsorption on ultrafine particles. Different adsorption isotherms were fit to the data and calculated for planar surface using the fitted isotherm constants. The measurements indicate that more than one layer of water molecules is adsorbed on both substances below the deliquescence relative humidity. Adsorption on NaCl was found to be stronger than on (NH4)(2)SO4.