Abstract: Ammonium is a typical pollutant encountered in municipal solid waste landfill leachate. However, there have been no researches about the diffusion behavior of ammonium in a geosynthetic clay liner (GCL) until now. A series of batch adsorption tests and diffusion specimen leaching tests are conducted to measure the distribution coefficient of ammonium onto bentonite. In addition, the diffusion tests are conducted to estimate the diffusion coefficient of ammonium in the GCL. Both the adsorption and diffusion coefficients are used in the POLLUTE v7.0 to simulate the mass transfer of ammonium through the GCL. The test results show that the distribution coefficients of ammonium onto bentonite is 0.017 L/g, and the diffusion coefficient through the GCL is 9.0×10^-11 m²/s. The simulated results obtained by the POLLUTE v7.0 show that the ammonium will break through the GCL about 30 years earlier when the diffusion behavior is considered.