We present the results of a Brillouin scattering experiment on aqueous mixtures of polyethylene glycol polymer cooled below their crystallization temperature. The shape of the registered spectrum changed considerably when the phase transition occurs. The Brillouin profile consisted of two peaks which has been interpreted as a result of the scattering of light on the local regions exhibiting different compressibility. Examination of the temperature dependence of the two Brillouin peak positions allows the identification of the specific microstructures present in the studied system. The existence of a complexlike hydration structure, stable against dilution and temperature agitation, was suggested. The nature and the behavior of the hydration complex is discussed in view of the earlier investigations in similar systems.