To study the hydration effect of poly(ethylene glycol) (PEG) and its dependence on the molecular weight, we report viscosity, compressibility, and quasi elastic light-scattering measurements on aqueous solutions of PEG with different mean molecular weight, M-w at different concentration and temperature values. In particular, ultrasonic technique allows to evaluate the hydration number for PEG samples at different polymerization degrees. The values deduced by ultrasonic technique are then compared with those deduced from viscosity data following the Linow and Philipp's model. PCS technique allows to obtain information on the hydrodynamic radius and its dependence on the polymer M-w at different temperature values.