Leakage across the parenteral vial/closure seal interface is quantitatively measured in terms of mass of gas per unit time using a differential pressure method of leakage measurement. With this test system, uncoated, Purcoat coated, and film coated closures are compared for their ability to seal nondefective and defective vial surfaces. Correlations are made between closure sealing performance and rubber viscoelasticity, closure coating material type and thickness, and crimped vial residual seal force.