Well-defined laboratory experiments have been carried out to investigate hysteretic dynamic effect in capillary pressure-saturation relationships for two-phase flow in homogeneous and heterogeneous (layered) porous media. Conceptually, the dependence of the capillary pressure curves on the rate of change of saturation (dS(w)/dt) is defined as the dynamic effect in capillary pressure relationship, which is indicated by a dynamic coefficient, (Pa s) and it determines the rate at which two-phase flow equilibrium is reached, i.e., dS(w)/dt = 0 where S-w and t are the water saturation and time, respectively. The dependences of on various fluid and porous materials properties have been studied in the context of drainage; but, there is limited study for imbibition and the hysteresis of -S-w relationships. As such, the emphasis in this article is on reporting -S-w curves for imbibition while also demonstrating the hysteresis in the -S-w relationships by comparing -S-w curves for drainage (previously reported) and imbibition (this study) in carefully designed laboratory experiments. Homogeneous sand samples composed of either fine (small particle size and lower permeability) or coarse (larger particle size and higher permeability) sand have been used for these experiments. Furthermore, a layered domain made of a find sand layer sandwiched between two coarse sand layers is used as a model of heterogeneous domain. The results of the study confirm that the -S-w relationships are hysteretic in nature and, as such, the speed to flow equilibrium should vary depending on whether drainage or imbibition takes place. At a particular water saturation, the magnitudes of the dynamic coefficient () are found to be generally higher for imbibition, which imply that the speed to flow equilibrium at the same saturation will be slower for imbibition. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 3958-3974, 2013