The influence of processing on the moisture resistance of epoxies and silicones used for packaging semiconductor devices is investigated. Conditions are assumed analogou to a capacitor charging up through a series resistance, and it is shown that this model applies with sufficient accuracy for optimally processed plastic compounds. Deviations indicate incorrect procecsing. Based on the model, characteristic values for moisture permeability and moisture capacity are defined and their variations with the processing conditions discussed. It is shown how the moisture capacity and moisture resistance influence semiconductor devices, and the relationship between moisture parameters and other characteristic values of the plastic is indicated.