A process for coating E-glass fibers with polystyrene-polyethyleneimine (PEi) core-shell particles was developed, and uniform monolayers of particles of 143 and 327 nm diameter were covalently bonded to the glass surface. The effect of the particle coatings on the mechanical properties of fiber-reinforced composites of poly(vinyl butyral) (PVB) was investigated. The interfacial shear strength (IFSS) was measured for specimens containing one to 20 fibers each using the tensile fiber fragmentation test, and significant enhancements were found, in particular for samples containing larger numbers of fibers. The smaller-particle (143 nm) coatings in the 20-fiber specimens produced approximately a 100% enhancement in IFSS over equivalent specimens with bare or aminosilane-treated fibers, while the 327 nm particle coatings produced only approximately a 25% enhancement. The greater effectiveness of the smaller particles was attributed, at least in part, to the larger effective interfacial area they provide and their relatively greater shell-to-core ratio, providing greater interphase stiffness. The greater enhancements achieved for the multi-fiber vs single-fiber specimens suggest that the coatings produce a more uniform fiber-fiber spacing and, therefore, a more thorough wetting of the fibers by the resin in the multi-fiber samples. Composites formed using fiber tows of 3200 fibers each showed more than a 100% increase in composite toughness and 35% increase in ultimate tensile strength as compared to samples with bare fibers due to the presence of the 143 nm particle coatings, and somewhat more modest increases for the 327 nm particle coatings. (C) Koninklijke Brill NV, Leiden, 2009