Toughening particles, comprising two radially alternating rubbery and glassy layers, were prepared by using sequential emulsion polymerization. The conditions which led to controlled particle size and morphology are discussed. A relatively new type of inert core shell particle [fly-ash (FA)] and surface-activated FA, by two different silane coupling agents, namely 3-aminopropyltrimethoxy silane (AMP) and vinyltriethoxysilane (VES)-based multilayered toughening particles, which radially comprise rubbery and glassy layers, were also prepared. The toughening particles were used with general purpose polyester resin (GPR) for making composite sheets. Formation of multiple layers in the core-shell particles and their morphology were confirmed by transmission electron microscopy (TEM). The mechanical properties such as tensile, flexural, impact, and hardness of the toughened GPR are discussed critically. The tensile fractured surfaces were studied by scanning electron microscopy (SEM). Thermal property such as thermogravimetric analysis (TGA) were also discussed. The composites were exposed to various adverse environmental conditions such as water, boiling water, salt water, acid, alkali, toluene, weather, and freezing-thawing for 30 days. The mechanical properties (viz. the tensile strength, tensile modulus, elongation at break, flexural strength, flexural modulus, impact strength and hardness of FA/GPR, FA.AMP core/GPR, and FA.VEScore/GPR) were studied before and after exposure to adverse environmental conditions. The results indicate that the mechanical properties of FA/GPR composite are improved by surface treatment of FA and their resistance to the various environmental stresses is also enhanced substantially on modification by toughening particles. (C) 2004 Wiley Periodicals, Inc.