Water adsorption was studied for spark discharge soot which is often used as a surrogate for atmospheric soot. Analysis by adsorption gravimetry and calorimetry showed that spark discharge soot exhibits specific features of hydrophilicity. Analysis of the isotherms and heats of water adsorption revealed the peculiarities of the mechanism of surface wetting related to significant changes in the soot microstructure. The first water adsorption/desorption cycle led to irreversible water adsorption and a decrease of the surface area available for water adsorption. Repeated water adsorption/desorption cycles demonstrated swelling phenomena related to the increase of the nitrogen surface area due to interparticle microporosity. The maximum effect of the transformation was observed under exposure to saturated water vapor following the microstructure stabilization. With decreasing temperature the amount of adsorbed water also decreased. Once emitted into the atmosphere the spark discharge soot particles will exhibit irreversible changes in surface area, porosity and hygroscopicity as a result of numerous humidity fluctuations until approaching a stable state of their microstructure due to long exposure to saturated water vapor. (C) 2003 Elsevier Ltd. All rights reserved.