Employing a modified T-Peel-Test it is shown that on covulcanization interfacial bonding occurs between a precured elastomer and a fresh rubber mixture if both consist of the same type of polymer (e. g. SBR1500). Crack initiation and propagation always proceeds in one of the polymer phases, not in the interface. Diffusion of curing agent into the precured elastomer gives rise to a step-like variation of crosslink density near the interphase, resulting in areas of net over-or undercuring in the vicinity of the interfacial border. Based on a concept of fracture mechanics the point of crack is interpreted with respect to the degree of crosslinking in the precured elastomer and the concentration of curing agent in the mixture. If incompatible systems are used, an increasing amount of adhesive fractures right at the interfacial border is observed due to a lacking or less prominent interphase.