This paper presents detailed Brillouin light scattering studies of the acoustic response of a Cs5H3(SO4)4xnH2O (PCHS) crystal in the vicinity of a superionic (superprotonic) structural phase transition of the first order. Just above the phase transition, splitting of the Brillouin doublet is observed. The ‘two-mode’ behavior of the longitudinal acoustic phonon can be explained by coexistence of phases at a structural phase transition of the first order. Above the phase transition, in the superionic phase, an additional doublet forbidden by the selection rules appears in a narrow temperature interval. It is concluded that an anomalous behavior of Brillouin light scattering can be attributed to the influence of dynamically disordered protons on the phonon subsystem.