Abstract The work described in this paper demonstrates the applicability of a new pore sizing technique to membranes, that of NMR spin-lattice relaxation measurements. Relaxation of the fluid magnetization in a saturated membrane is related to pore size. This is due to different surface and bulk fluid relaxation rates being coupled with fast diffusional exchange between the regions. This NMR technique is not pore shape dependent in the pore size range of interest for membranes, and avoids many of the problems inherent in traditional techniques. Advantages of this approach include: (1) a wide range of pore sizes can be analysed (0.5 nm to > 1 μm), (2) the method is non-destructive, (3) it is immune to network/percolation effects, and (4) it can be used with wet materials. The average pore size and pore volume distributions of five Millipore mixed esters of cellulose membranes (nominal radii 12.5-2500 nm) were determined via NMR and the results compared with those from mercury porosimetry, nitrogen adsorption/condensation, and flow permeability/bubble point. The NMR technique was seen to compare favorably with the other methods.