A fraction of a three-component system is defined as a thermodynamic entity that groups two components. In this work, the thermodynamics of fractions is developed. The Kern-Weisbrod method has been proposed for two-component systems, but by considering the system as fractionalized, this method can also be employed to calculate the specific partial quantities of the components of a three-component system. This method is used to study the swelling process of functionalized polymeric particles. A series of copolymers of butyl acrylate (BuA) and methyl methacrylate (MMA) functionalized with 0, 5, 10, 15, and 20 wt % of acrylic acid (AA) was synthesized by emulsion polymerization. A thermodynamic study of the polymeric particle swelling as a function of the acrylic acid content and the temperature was carried out. Specific partial volumes, compressibilities, and expansibilities were calculated using density and speed of sound measurements. It was found that the hydration behaviour of carboxylic and nonpolar groups of the polymeric particles is thermodynamically driven in accordance with the Gibbs-Duhem equation.