A wealth of information about the structure of porous materials is contained in the experimental data of mercury intrusion and retraction obtained from mercury porosimetry. Proper interpretation of such data requires the use of a model capable of describing the phenomena of capillary pressure hysteresis and mercury entrapment. This work advances the concept of mixedpercolation on cubic lattices representing pore bodies, connected through pore throats, as a general model for the study of hysteresis and entrapment observed in the course of mercury refraclion in porosinetry. The dominant phenomena of local capillary instability (snap-off of mercury threads, piston-type intrusion and re(raction), aflecting the movement of mercury-air interfaces in pore throats and pore bodies, are accounted for and related to the macroscopic capillary pressure-saturation behavior by means of computer simulation. Local correlations among pore throat and pore body sizes, as well as microscopic heterogeneities due to spatial correlations among the pore bodies are also accounted for in the model. The simulator is used to study the dependence of mercury porosimetry curves on pore throat and pore body size distributions and spatial correlations, as well as on contact angle. It is shown that mercury retraction can be successfully modeled as the deterministic outcome of the competition between two simultaneously occurring percolation processes, snap-off in pore throats (bond-percolation) and pistontype retraction from pore bodies (site-percolation). O 1993 Academic Press. Inc.