A laboratory setup and a procedure for measuring the volume coefficients and mean cross sections of extinction, scattering, and absorption of soot particles in the medium of saturated water vapor are described. A method for hydrophilization of the surface of soot particles, which makes it possible to obtain model objects with specified hygroscopic properties, is presented. The processes of transformation of soot particles are analyzed with the use of data of electron-microscopic investigations on the basis of the fractal approach. The structural parameters of hydrophobic soot are shown to depend on the conditions of moistening, whereas hydrophilic particles are subject to substantial structural changes indicative of their watering. Investigation of the coagulation of soot particles with drops shows that hydrophobic particles form a weakly bound system; they coagulate on the surface of drops and cause no changes in the structure of soot aggregates. During the coagulation, hydrophilic soot particles penetrate inside a drop and irreversibly form a mixed system.