Hollow-particle latexes were prepared according to the following stages: (1) the preparation of the methyl methacrylate-methacrylic acid (MAA)-ethylene glycol dimethacrylate copolymer (1) latex, (2) the preparation of a shell (II) based on polystyrene or styrene-acrylonitrile-divinyl benzene copolymer polymerized onto copolymer (1! particles, and (3) the neutralization of the core (I) carboxyl groups with a base (NH,OH or NaOH) at temperatures close to the glass-transition temperature of the polymer (II). The neutralization resulted in the expansion of the particles and formed water-filled hollow particles. The microspheres had an overall diameter of 460 - 650 nm and a hollow diameter of 300-450 nm. Rheological studies and particle size measurements by transmission electron microscopy and dynamic light scattering of the copolymer (I) latex indicate that the maximum particle swelling occurred at an approximately equimolar MAA/base ratio. It was found that even without the neutralization of the MAA units, a small hollow formation in the latex particles occurred during stage 2 because one volume of the copolymer (1) retained about 8 volume parts of water. It was also discovered that the final hollow-particle geometry after neutralization depends on the shell copolymer thickness and type as well as on the conditions during stage 3, that is, the time, temperature, base type, and concentration. The opacifying ability of the synthesized hollow particles was investigated in latex coatings. The opacifying ability values were generally in agreement with the hollow-particle geometry. The only exception was related to the copolymer (I)/copolymer (II) ratio. The maximum hollow volume was obtained at this value equal to 1/8, whereas the highest opacifying ability was observed at 1/10.