The major objectives of this paper are to investigate how the form and hysteresis of mercury injection, withdrawal and reinjection capillary pressure curves are affected by the geometry of pores and their connections in samples of sedimentary rocks and also in artificial and theoretical pore-network models. In particular, those aspects of pore systems which may influence trapping of mercury during pressure reduction and withdrawal are considered. These are: pore to throat ratio, throat to pore coordination number or connectivity and the types and extent of random and non-random heterogeneities within the system. These aspects of pore systems influence the threshold pressure and the gradient of injection curves as well as the gradients and degree of hysteresis displayed by withdrawal and re-injection curves. Such curves are useful in interpreting pore geometry and give information which is valuable in assessing multiphase fluid behaviour in oil and gas reservoir rocks. In the case of water displacing oil or gas, in a strongly water-wet system, the trapping of oil or gas is controlled mainly by capillary forces and a direct analogy with the air-mercury system is possible.