The mechanical and physicochemical properties of epoxy compounds containing different amounts of a series of fillers have been investigated. The strength characteristics of compounds with reinforcing fillers improve, starting from very low concentrations, pass through a maximum, and then fall at large concentrations. In the case of nonreinforcing fillers, the strength falls, slowly at first, and then, beyond a certain critical concentration, much more rapidly. Electron microscope studies have shown that reinforcement of the resin is associated with deep-seated changes in supramolecular structure, which passes from globular to partially fibrillar when the resin is cured in the presence of fillers. On the basis of the studied concentration laws a method is proposed for characterizing the reinforcing effect of fillers with respect to the particular property tested and a given production technology.