The pore structure of fragments of steam reforming catalyst ring-shaped pellets was characterised so as to simulate a larger particle using larger pore networks comprising millions of pore segments. The pore size distribution generated for a 3-D stochastic network was deduced from the experimental mercury penetration curve. The penetration processes which take place within the pore network at different pressures can be visualised by randomly slicing irregular 3-D pore networks to create 2-D "Virtual reality slice" images. The experimental visualisation technique of low-melting alloy impregnation was applied to the larger ring catalyst fragments at several pressure levels, in order to investigate the effect of size, the fragment geometry, and a suspected skin effect on the mercury penetration curve for these particles. The experimental scanning electron microscopy images, obtained from polished sections of a large 5-mm, ring fragment, can be compared directly with the 2-D virtual slice images from randomised 3-D pore networks for direct quantification of the pore structure using image analysis techniques. (C) 2003 Elsevier Science Ltd. All rights reserved.