The structural, thermodynamic, and dynamical properties of a single ammonium ion, NH4+, in liquid water were investigated using molecular dynamic simulation techniques. Many-body polarizable model potentials were employed to describe the molecular interactions. The water molecules were found to form well-defined hydration shells with a preferred orientation around the NH4+ ion as expected. We also found that the average water dipole moment is enhanced in the first hydration shell. The translational and rotational motion of NH4+ was examined via the velocity, angular-momentum, and reorientation-autocorrelation functions. The computed rotational diffusion coefficients of the NH4+ ion in water, which were determined from the angular momentum autocorrelation function and the angular mean-square displacement, are 0.093 and 0.067x10(12) rad(2)/s, respectively. These results are in good agreement with the experimental nuclear magnetic resonance value of 0.075x10(12) rad(2)/s. (C) 2003 American Institute of Physics.