A series of functionalized multilayer latex particles of P(butyl acrylate-methyl methacrylate-acrylic acid) was synthesized. The total acrylic acid concentration inside particles was varied from 0 to 25 wt.% and the number of particles in the system was held constant. The polymeric particles were studied by means of specific partial volume and specific partial compressibility at infinite dilution, which were calculated from density and sound speed measurements. From the behaviour of the partial properties at infinite dilution of the polar and non-polar groups of the polymer chain, it was concluded that the particle swells at low acrylic acids content due to the electrostatic repulsion between the carboxylic groups of the polymer chain. From 15 wt.% of acrylic acid up, the particle swells due to the electrostatic repulsion and to the incorporation of water molecules in the bounds of the particle layers. In this way, the hydration is non-homogeneous in the interior of each layer. These results are in agreement with the behaviour of the hydrodynamic radius determined by the quasi-elastic light scattering technique. (C) 2008 Elsevier Ltd. All rights reserved.