The influence of thermal processing and storage conditions on oxygen ingress in multilayer retortable trays containing ethylene vinyl alcohol copolymers (EVOH) and polypropylene (PP) was investigated. Oxygen ingress in PP/EVOH/PP trays during thermal processing was, found to depend on processing conditions, such as retort temperatures, process times and oxygen partial pressures. Temperature and relative humidity (RH) conditions during storage strongly affected the oxygen permeability of the EVOH-containing trays. Oxygen ingress in PP/EVOH/PP trays was minimal during one year of storage at 21.1°C, 60% RH, but increased substantially after 100 d storage at 32.2°C, 75% RH. A correlation was also found between oxygen permeation during storage and the hydration of EVOH-containing trays during thermal processing. The moisture sorption isotherms of three EVOH films (EF-E15, EF-F15 and EF-XL15) were determined at three temperatures (25, 35 and 45°C) and over a range of RH from 23 to 100%. As the EVOH films sorbed moisture, their thermal and mechanical properties were affected substantially. Glass transition temperatures of the EVOH films decreased with increasing RH. Mechanical property measurements indicated that both the tensile modulus and yield strength of the EVOH films decreased with the increase in RH. In contrast, elongation at failure increased as RH increased, indicating that the polymer became more ductile at high RH. The oxygen and limonene barrier properties of EVOH and nylon 6,6 films were determined at various temperatures and RH. The transmission rates of oxygen and limonene through EVOH films were markedly affected by temperature and RH. In general, the EVOH films had better limonene barrier properties than the nylon 6,6 film. Finally, a mathematical model and computer program were developed to predict oxygen permeation into multilayer plastic packages exposed to known processing and storage conditions. Model and program development were based on the GAB equation describing moisture sorption of the EVOH films, and on semi-empirical equations developed to describe the oxygen transmission rates of the EVOH films, and water transport through multilayer plastic packages.