The present invention relates to an integrally blow-molded bag-in-container (2) having an integrally blow-molded bag-in-container wherein the same polymer is in contact on either side of the interface between the inner (11) and outer layers (12). It also concerns a preform (1, 1′) for blow-molding a bag-in-container, having an inner layer and an outer layer, wherein the preform forms a two-layer container upon blow-molding, and wherein the thus obtained inner layer of the container releases from the thus obtained outer layer upon introduction of a gas at a point of interface between the two layers. The inner and outer layers are of the same material.