Volume or mass and viscoelastic changes in Os(byp)(2)ClPyCH2NH2-poly(allylamine) hydrogel (Os-PAA) cross-linked with glucose oxidase (GOx), have been measured by studying the electroacoustic impedance at 10 MHz. Film thickness and shear modulus have been obtained from imaginary and real components of the EQCM acoustic impedance in transient (cyclic voltammetry and chronoamperometry) as well as steady state oxidation and reduction of the hydrogel. Swelling of the film during Os-II oxidation resulted in mass uptake and decrease of shear modulus. These quantities depend only on the degree of polymer oxidation and not on film history or kinetics. Gravimetry beyond the Sauerbrey mass limit could be realized by use of Martin's viscoelastic model to treat the quartz crystal impedance.