On the basis of the nuclear magnetic resonance (NMR) relaxation of imbibed water, we evaluated the interparticle and intraparticle pore sizes in packed beds of silica materials of known particle sizes and microporous structure. The NMR relaxation distribution is scaled by the surface relaxivity parameter &rgr;, which incorporates a surface area to volume ratio (So/Vo) term, to yield a corresponding pore size distribution. The NMR-derived pore sizes of nonporous silica sand agreed with the expected interparticle pore sizes estimated from the morphology of a packed bed of spheres of comparably sized particles. The NMR-derived intraparticle pore size for porous silica was also in good agreement with reported values for the silica materials studied. Scaling of the NMR relaxation corresponding to interparticle water by the same surface interaction parameter to yield interparticle pore size in high-surface area porous silica material, however, grossly underestimated interparticle pore size. In these high-surface area materials the intraparticle micropores provide a higher contribution to the N2 measured surface area relative to the contribution from interparticle macropores. When the NMR relaxation method was used to evaluate the pore space in the Borden Aquifer material, the NMR-derived pore sizes agreed with those observed in scanning electron micrographs as well as pore sizes estimated from the morphology of packed beds of comparably sized particles. For soils and aquifer materials of low to moderate surface area the NMR-derived porosity determination may be used to adequately evaluate both solute transporting and sorbing pore sizes. ¿ 1997 American Geophysical Union