A primary concern in microelectronic packaging is the role of moisture induced failure mechanisms. Moisture is a multidimensional concern in packaging, having an adverse effect on package reliability by introducing corrosion, development of hygro-stresses, and deterioration of polymer interfaces within the package. In this paper the effect of moisture on the interfacial adhesion of two no flow underfill materials with a commercially available soldermask coated FR-4 board is experimentally determined. Bilayer specimens with prefabricated interface cracks are used in a four point bending test to quantify the interfacial fracture toughness. Two groups of test specimens of varying underfill thickness were constructed. The first group was fully dried while the other was moisture preconditioned at 85°C/85%RH for 725 hours. The results of this study show that the interfacial toughness is significantly affected by the presence of moisture.