Coagulation of polydisperse detonation nanodiamond (DND) hydrosol containing primary aggregates with a prevailing average size in a range of 20–200 nm has been studied experimentally and theoretically within the framework of the classical and extended DLVO theory as depending on the concentrations of an indifferent electrolyte (NaCl) and potential-determining ions (pH). It has been shown that the surface of DND particles is charged due to the ionization of amphoteric hydroxyl and acidic carboxyl groups located on it. The isoelectric point of the detonation nanodiamond particles has been found to correspond to pH 7.5. It has been revealed that the main stabilizing factor of the DND hydrosol is electrostatic. It has been shown that the stability and coagulation of the sol can be described within the framework of the extended DLVO theory using the effective constant for primary porous aggregates and taking into account the initial polydispersity of the DND particles.