Through-diffusion experiments in Boom Clay have been performed with uncharged molecules: tritiated water (HTO) and dissolved gases of different size (He, Ne, H2, Ar, CH4, Xe and C2H6), allowing information to be obtained on the relationship between the diffusion coefficient and the molecular size (characterized by a 'kinetic diameter' of the molecules). Experiments have been performed on both clayey and silty Boom Clay samples, to scope for the changes induced by grain size variations on the diffusion process. Experiments on clay cores taken perpendicular as well as parallel to the bedding plane have also been executed, providing additional information on the anistropy of the diffusion process. Empirical relations are proposed to capture the observed decrease of both the diffusion coefficient in water and the effective diffusion coefficient in the Boom Clay porous medium as a function of molecular size. In the same way, the behaviour of the geometric factor G as a function of size is estimated. Although silty samples have a noticeably higher hydraulic conductivity than clayey samples, the difference in diffusion coefficient is less obvious. The anisotropy factor is roughly the same for all investigated components, with an average value of 1.5.