Sealant adhesion to different substrates is often assessed by using a 180° peel test. It is known that the peel force is a function of the viscoelastic response of the adhesive joint coupled with the intrinsic strength of the interphase. Measurement of the fracture energy as a function of straining rate can yield material properties by separating the bulk effects from the interphase strength. The results of this study highlight some important deficiencies of the ASTM peel test method. In particular, the fracture energy of a silicone sealant to glass and aluminum was measured at different rates, peel thicknesses and sealant moduli using the 180° peel test. It was determined that the sealant failed cohesively when tested on glass. For instances of cohesive failure, a plot of fracture energy vs. strain rate fit a power law model. The rate dependence of the fracture energy was shown to be proportional to the amount of uncrosslinked polymer in the sealant and this suggested an increase in dissipation. It was further shown that as the modulus of the sealant decreases the fracture energy increases. However, at low strain rates, the trend reverses and the high modulus sealant has better adhesion to glass than some of the low modulus sealants. Tests on specimens with aluminum substrates failed in accordance with specific test conditions: adhesive failure was more likely to occur when the strained thickness was small; when the strain rates were slow; or when the modulus was high. This study clearly demonstrated that peel testing at one rate and thickness can not adequately compare one sealant to another.