The effect of cocrystallization on the welding behavior of semicrystalline polymers was studied by means of T-peel testing at room temperature, using ultrahigh-molecular-weight polyethylene (UHMWPE) as a model polymer. Solution-cast films of UHMWPE have a very special morphology, consisting of regularly stacked, 107 angstroms thick lamellae, which exactly double in thickness upon annealing for 15 min below the melting point at 125 °C. This lamellar doubling process was used to introduce a well-defined amount of cocrystallization across the interface, by annealing two stacked, completely wetted solution-cast films. It was found that doubling of the lamellae across the interface enhances the peel energy to such a level that the films could not be separated anymore. By contrast, reference samples, in which cocrystallization across the interface was prohibited by `preannealing' one side of the film, could still be separated easily. Therefore, it is concluded that cocrystallization across the interface is extremely efficient in enhancing the adhesive fracture energy.