A set of bench-scale experiments were conducted to estimate the contribution of air diffusion to pressure decay in an intact immersed membranes. The apparatus consisted of a pressurized vessel, an air compressor, a pressure regulator and an inverted water-filled burette for measuring the volume of diffused air. The results obtained from air diffusion measurements allowed for consistent determination of membrane's true bubble-point pressure and the size of the largest pore. The results also indicated that air flowrate due to diffusion through an intact wetted membrane is relatively constant for a specific membrane with the same surface area and water temperature. Furthermore, it was demonstrated that the pressure decay in an intact membrane was primarily due to air diffusion through water-filled pores. Therefore, the dilution effect observed during pressure decay tests on membranes with large surface area can be mainly attributed to air diffusion through intact pores. Based on the experimental results a mathematical model was developed that estimates the amount of air diffusion through an intact wetted membrane as a function of applied pressure. The model also estimates the contribution of the diffusive airflow to pressure decay test. It is proposed that subtracting this contribution from the measured pressure decay tests can lead to more sensitive tests and better estimates of true log reduction values for microorganisms. This paper presents the development and verification of the mathematical model for estimating diffusion air flowrate through an intact ZeeWeed(R)500 membrane. (C) 2004 Elsevier B.V. All rights reserved.