Ammonia diffusion in H-ZSM-5 (Si/Al = 20) has been studied by the PFG NMR method for different ammonia loadings at 298 K. The results obtained indicate that the self-diffusion coefficient of ammonia changes by up to two orders of magnitude if ammonia loading is varied by a factor of only 2 between 1.6 and 0.8 mmol/g. This unusually strong loading dependence is attributed to the strong interaction of ammonia molecules with the limited number of acid sites of the H-ZSM-5 framework.