Hybrid inorganic/organic membranes, often called mixed matrix membranes, are composed of (inorganic) molecular sieves dispersed in a continuous (organic) polymer matrix. Recent studies have demonstrated that mixed matrix membranes have the potential to exceed the performance of conventional polymer membranes while maintaining their processability. Because water vapor can be present in air and in natural gas feeds, it is an ideal candidate for studying the effects of strongly sorbing minor components on mixed matrix membranes. Water vapor has been shown to cause complex but predictable effects on mixed matrix materials. In this article simple models are proposed to describe the behavior of mixed matrix membranes with humidified feeds to assist in interpreting observed performance. This research has shown that mixed matrix membranes composed of zeolite 4A dispersed in poly(vinyl acetate) are adversely affected by water adsorbed in the pores of zeolite 4A. On the other hand, a novel, more hydrophobic molecular sieve, SSZ-13, appears to be an ideal candidate for mixed matrix membranes because it has improved transport properties over zeolite 4A and is less affected by sorbed water. (C) 2003 Wiley Periodicals, Inc.