We report the results of Brillouin scattering measurements performed on a series of alkali metaphosphate glasses and melts including the mixed alkali composition. Using independent viscosity data, we have applied a time- temperature superposition approach to obtain from the relevant features of the Brillouin spectra the relaxation characteristics of the complex longitudinal elastic modulus. A mixed alkali effect is evidenced in the Brillouin scattering which occurs in the gigahertz range. We find a relaxation process that can be described by a Kohlrausch function with β ≃ 0.30 for single alkali compositions, but is narrower (β ≃ 0.40) at the mixed alkali composition.