Coagulation kinetics of homodisperse hydrosols containing spherical particles of selenium have been studied. Two independant experimental methods were used. One is based on direct observation of the sols under the ultramicroscope, the particles contained in a given volume being counted. The second method is a spectrophotometric one, based on Mie's theory. It has been possible to reproduce and to interpret the abnormally low value of R/a (R being the distance between the centers and a, the radius of the particles) observed by Kruyt and van Arkel for the rapid coagulation of selenium hydrosols. The Schultze‐Hardy rule, interpreted theoretically by Verwey and Overbeek was confirmed quantitatively. The potential values calculated by Reerink and Overbeek's equations are far too low. Notwithstanding the use of spherical particles of identical size, there was a considerable dispersion of the Van der Waals constant (2.10−14 to 25.10−14 ergs) obtained under various conditions.