Brillouin light scattering from acoustic phonons has been used to investigate the elastic properties of Nation 115 membranes in the GHz range of frequencies. Measurements were carried out as a function of temperature from 0 to 60 degrees C on samples with different hydration. Detection of longitudinal bulk waves travelling either parallel to the free surface or at an angle with respect to the normal to the surface enabled us to evaluate the refractive index and the longitudinal modulus of the membranes. A marked decrease of the latter with increasing either the temperature or the hydration degree was found. Moreover, a comparison of the above results with those obtained from measurement of the Young's modulus (E) in the quasi-static regime (stress-strain curves) shows that the high-frequencies values of E are an order of magnitude larger than in quasi-static conditions, reflecting the viscoelastic character of Nation. (C) 2009 Elsevier B.V. All rights reserved.