The dynamical properties of protonic conductor K3H(SeO4)(2) were examined by Brillouin scattering. Both longitudinal-acoustic modes (c(33) and c(11)) and a transversal mode (c(44)) monotonously depend on temperature, and no soft mode was observed. A central mode with a width about 6 GHz was observed above phase-transition temperature, T-C. The central mode has a symmetry E-g corresponding to the rotational mode of SeO4 tetrahedra. The E-g mode is compatible with a zone-boundary soft mode, L-3, which accounts for the symmetrical change from R (3) over barm m to C2/c. The intensity of the central mode declines in the vicinity of T-C. This shows that chaotic rotational motion changes into a zone-boundary soft mode with decreasing temperature.