Silica with an average particle size of 125 nm was prepared by the sol-gel reaction of tetraethoxysilane with a base catalyst and then modified with a vinyl-functionalized silane [gamma-methacryloxypropyl trimethoxysilane (MPS)]. The composite later, with the modified silica as the core and poly(n-butyl acrylate-co-acrylic acid) [poly(BA-co-AA)] polymer as the shell, was synthesized by a semicontinuous emulsion polymerization and used as a pressure-sensitive adhesive. The structures were characterized by Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, elemental analysis, and transmission electron microscopy. The particle sizes of the silica and MPS-modified silica (MPS-silica)/poly(BA-co-AA) composite latexes were determined by dynamic light scattering in a semicontinuous emulsion polymerization online. The monomers of n-butyl acrylate and acrylic acid grew around the MPS-silica particles without significant secondary nucleation, and the composite latexes exhibited a core-shell structure with the modified silica particles enwrapped in poly(BA-co-AA). To compare the adhesion properties, including the loop tack force, peel strength, and shear resistance, by Federation Internationale des Fabricants et Transformateurs d'Adhesifs et Thermocollants sur Papiers et Autres Supports test methods, poly(methyl methacrylate-co-allyl methacrylate)/poly(BA-co-AA) latex and the full poly(AA-co-AA) latex were prepared with the same particle size and the same emulsion polymerization process. The shear resistance of the composite latex film greatly increased after the addition of the silica to the core of the poly(BA-co-AA) latex. The relationships between the adhesive properties and the different structures in the core components were examined. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3113-3124, 2009