<p>Peel tests are commonly used to characterize the performance of adhesive tapes. The force required to peel a tape from a substrate depends on not only interface adhesion but also mechanics of the tape. Typically, adhesive tapes consist of a stiff backing film and a layer of adhesive material that is soft and viscoelastic. While mechanics of the backing film has been extensively studied, mechanics of the soft adhesive layer is less understood. In this work, finite element simulations are carried out to study large deformation of the soft adhesive layer during 90-degree peeling and its implication on the peel force. We find that debonding can occur ahead of the peel front when the peel front is still adhered to the substrate. This phenomenon, referred to as “interfacial cavitation”, causes the peel front to advance in a stepwise manner despite that a constant peeling velocity is prescribed. Consequently, the peel force follows an oscillatory history resembling the “stick–slip” behavior widely observed in peel tests. Further investigations reveal that interfacial cavitation originates from a non-monotonic distribution of interfacial traction ahead of the peel front. Moreover, emergence of interfacial cavitation can be controlled by three factors: interfacial slip, adhesive layer thickness and peeling velocity. These results can provide insights towards designing adhesive tapes with desired adhesion performance or release mechanisms.</p>