The pulsed field gradient (PFG) NMR method is applied to study molecular diffusion in beds of spherical silica particles. From the observed transport properties it may be concluded that the internal pore system of the silica particles gives rise to two different modes of molecular migration : restricted motion with molecular mean square displacements of the order of 700 nm, and unrestricted motion with effective diffusivities decreasing with increasing loading over nearly 2 orders of magnitude. In PFG NMR measurements with nonporous silica, the molecular mobility in the space between silica particles and the topology of the space between these particles were investigated. In these experiments a slowly diffusing macromolecule, poly(dimethylsiloxane) (PDMS), is shown to be a most effective probe molecule for dynamic imaging.