Compressibility of boundary layers in polymer composites can be evaluated acoustically by applying a two states model based on a solvation model. In this tutorial paper, we introduced the method to characterize the elastic properties of the boundary layer, as well as features of acoustic characterizations for materials. Polyvinylchloride (PVC) and styrene-butadiene rubber (SBR) were used as polymer matrices. Calcium carbonate (CC) and carbon black (CB) were used as fillers and these were dispersed in the matrices. Velocities of elastic waves propagating in the PVC/CC and SBR/CB composites were measured by using two bulk wave velocity measurement systems. Acoustic images and surface wave velocities of the composites were measured with an acoustic microscope. Amount of boundary layers for the PVC/CC and SBR/CB composites were determined from the acoustic image and from the thermal gravimetric curve, respectively. The partial specific volume of those particles was almost same as the specific volume in each particle. Values of the partial specific adiabatic compressibility of the CC and CB were 1.4±0.7×10?10 Pa?1 and ?0.5±0.5×10?10 Pa?1. These values suggested that the boundary layer in the PVC/CC composite was softer than the matrix and that in the SBR/CB composite was harder than the matrix.