Mechanical reliability of epoxy molding compounds in plastic packages of integrated circuits (IC) devices is affected to a great extent by the materials ability to absorb moisture. Accordingly, the objective of the study is to evaluate the effect of moisture sorption on the mechanical properties of the compounds. The diffusion of water in compounds filled with silica and alumina nitride particles has been studied experimentally for systems with moderate and high concentration of fillers. The analysis of the weight gain profiles has shown that the process of moisture diffusion is non-Fickian and that the moisture weight gain depends on the specimen’s relative humidity and the concentration of fillers. As to the hygro-thermal (swelling) stress, caused by moisture diffusion, we found that this stress is very low (its average value does not exceed 662 psi). We showed also that moisture diffusion leads to an appreciable decrease in the compound’s strength and to a substantial increase in the material’s plasticity. The obtained results can be helpful in the analysis of the mechanical behavior of epoxy molding compounds used in electronic packaging.