This work investigated the relationship between the NCO:OH ratio of the polyurethane (PU) adhesive of thermally treated aluminum substrates and the peel resistance of the obtained flexible laminate. Bicomponent PU-based commercial adhesives were combined with three different proportions of an isocyanate prepolymer and a biopolyol to vary the NCO:OH ratio. The aluminum substrates studied were commercial aluminum films deposited onto a bioriented polypropylene (BOPP) film via vacuum evaporation. The aluminum substrates were thermally treated at 60 degrees C under an atmosphere of air using a conventional laboratory oven for different aging times. The aluminum substrate surfaces were characterized using X-ray photoelectron spectroscopy to evaluate their composition. Atomic force microscopy was employed to evaluate their morphology and contact angle to calculate their surface energy. The adhesive was deposited onto the aluminum substrates and a polyethylene film via manual lamination. The resulting samples were subjected to peel tests. The results showed a reduction in peeling resistance with increasing aging times. Meanwhile, a decrease in the NCO:OH ratio led to a reduction in the peeling resistance for shorter aging times.