Hydration studies of n-hexane soot particles in the relative H2O pressure range 0.33--0.52 have revealed the nature of the processes between water molecules in the vapor phase and primary reaction sites at the particle surface. Initially, about 40% of the carbon-oxygen functionalities (most likely the carboxylics) on freshly prepared soot reacts irreversibly with water vapor, while at P/P0≥0.48, the adsorption follows the Dubinin-Radushkevich equation. Determination of these parameters enables calculation of surface coverage at limiting adsorption and at the chemisorption limit for all materials studied. Limiting surface coverage of fresh soot is consistent with a 50% oxygen coverage determined earlier; it increases to about 100% for nitrated soot, which is the most extensively hydrated. Hydration has been shown to increase with soot aging, a phenomenon which has revealed an important role of physisorbed O2 in hydration. Trace-metal incorporation at the 30--110 ppm level significantly increases particle hydration and demonstrates a role of metal centers in the process. ¿ American Geophysical Union 1996