A new structural model for the interpretation of mercury porosimetry data for samples having a bimodal pore size distribution is presented. The morphology of the macropore network of a porous solid has been studied using a combination of mercury porosimetry and NMR techniques. The model consists of a lattice-based network of pore bodies and pore necks. The model enables assessment of the spatial geometric distribution of the directly accessible macropore network of a porous material having a bimodal pore size distribution. A methodology that can be used to determine the spatial distribution of the critical pore neck diameters controlling access to the macroporous void space has also been described. This method was used to provide a map of the local critical macropore neck distribution over macroscopic length scales for an alumina tablet that is not possible with other techniques.