Different types of polymer nanocomposites on the base of polystyrene, polymethylmethacrylate, and polycarbonate with alumina and SiO2 nanoparticles and carbon nanotubes have been studied. Miniaturized, microdimensional samples were used, enabling a good control of morphology and distribution of particles by means of transmission and scanning electron microscopy. Special preparation techniques had been applied, which resulted in a very good dispersion of the nanoparticles. Using these materials with really nanosized particles of a few 10 nm in size the effect on toughness enhancement could be studied without agglomerates as they often appear in the generally used large samples. Micromechanical mechanisms were studied in detail by TEM and SEM investigations of deformed samples. A "nanoparticle modulated crazing" could be detected as a toughness enhancing effect. (c) 2007 Wiley Periodicals, Inc.