The NMR pulsed field gradient technique is applied to measure the self-diffusion coefficients of water, methanol and benzene molecules adsorbed on different active carbons. It is found that under the influence of the adsorbent, molecular mobilities are reduced by up to two orders of magnitude. The observed concentration dependence of the self-diffusion coefficient of water can be explained on the basis of the well-established model of water adsorption by primary adsorption sites. Irrespective of the various nature of the active carbons studied, for both water and methanol the self-diffusion coefficients are found to increase with increasing pore width. For most of the active carbons investigated, molecular mobilities decrease in the sequence water, methanol, and benzene (i.e., in the sequence of increasing molecules size). Only for the active carbons with the largest pore widths are the self-diffusion coefficient of methanol greater than that of water. Moreover, for these adsorbents, the diffusivities of both adsorbates are by the same factor lower than those of the neat liquids. This indicates that for active carbons with large pore widths the reduction of translational molecular mobility is mainly due to the influence of tortousity.