By curing of in water dispersed droplets of an aromatic or aliphatic epoxy resin, respectively, glassy or rubbery thermosetting epoxy spheres can be prepared in a relatively easy way. The rubbery epoxy particles can successfully be applied as a toughening agent for glassy epoxy matrices. The advantage of these preformed modifiers is the control over the final morphology as the size and concentration of the dispersed rubber phase can be chosen independently. The improvement in fracture toughness and the morphological features of the fracture surfaces are identical to standard, but also more complex, toughening routes, such as the application of liquid rubbers [Sultan JN, Laible RC, McGarry FJ. Appl. Polym. Symp. 1971;16:127; Sultan JN, McGarry FJ. Polym. Eng. Sci. 1973;13:29]. The use of this new class of rubber modifiers is not restricted to glassy epoxy resins, but may also be applied to produce impact-modified thermoplastics with a predetermined particle size and rubber content.