The nonlinear behavior of proton conductivity in phase III of (NH4)(3)H(SeO4)(2) (abbreviated as TAHSe) along the c-axis is interpreted in terms of mixing of two phases in the transition region between phase III and phase II. The pure phase III of TAHSe is simulated by means of one-dimensional model whose evolution is driven by the kinetic Monte Carlo method. Experimental data at moderate pressure are used to ensure the quality of fitting of model parameters. The interplay of lattice vibrations and the hydrogen bond length is demonstrated to be the key physical factor responsible for the proton diffusion. (C) 2017 Elsevier B.V. All rights reserved.