The hydration and viscoelastic properties of synthetic polymer brushes on a substrate were evaluated using a 27 MHz piezoelectric quartz-crystal microbalance based on admittance analysis (QCM-A). The (-Delta F-water)/(-Delta F-air) values, which indicate the hydrodynamic water (bound and vibrated water) ratio per unit polymer mass, and the Delta D-water/ (-Delta F-air) values, which indicate the viscoelastic properties (energy dissipation per mass), were obtained for low- and high-density brushes of each polymer. These values indicated: that the low-density polymer chains had different properties, which depended on the chemical features of each polymer. In, contrast, these values were similar for the high-density brushes of each polymer, indicating that the polymers were dehydrated and more elastic due to limited spate:for movement of the chains with the solvent. These mechanical properties are expected to relate to functions such as surface friction and biocompatibility.