The Boeing sol-gel process (Boegel-EPII) is a surface preparation method for metallic substrates for adhesive bonding and painting applications. This paper describes an investigation into the effect of processing conditions on adhesion strength and durability of a sol-gel reinforced, rubber toughened epoxy/aluminum joint. Using an asymmetric double cantilever beam (ADCB) wedge test, the adhesion performance of the sol-gel reinforced epoxy/aluminum joint in a humid environment was measured as a function of sol-gel processing conditions. The sol-gel drying time, concentration and drying humidity all have an effect on adhesion performance. Prolonged drying led to a decrease in fracture energies. The critical and threshold fracture energies show different trends as sol-gel concentration varies, and better adhesion performance was observed for sol-gel dried at higher humidity compared to lower humidity. The failure modes and mechanisms were studied by XPS and SEM. Analysis of locus of failure revealed that the observed trends for adhesion performance can be explained in terms of interdiffusion of the sol-gel film and epoxy. The diffusion of the epoxy into the sol-gel layer is hypothesized to strongly depend on the degree of condensation of the sol-gel film and is directly affected by the sol-gel processing conditions. (C) Koninklijke Brill NV, Leiden, 2008