Porous anodic aluminum oxide (AAO) templates with 20, 35, and 60 nm cylindrical pores were prepared and subsequently infiltrated with poly(ethylene oxide), PEO, polyethylene, PE, and polyethylene-block-polystyrene diblock copolymers, PE-b-PS, of various compositions. The crystallization of the nanocylinders within the AAO templates was studied by differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD). A clear change from heterogeneous to homogeneous bulk nucleation was detected in the PEO before and after infiltration within a 20 nm template, respectively. The homogeneously nucleated nanocylinders needed extreme supercooling in order to crystallize, and their low crystallization temperature was successfully correlated with the volume of the crystallizing phase. 2D-WAXD measurements demonstrated that the PEO chains within the crystals formed inside the nanocylinders, preferentially orient perpendicular to the cylinder aids. This chain orientation is probably due to the easier crystal growth propagation along the cylinder length (200 mu m for the templates employed to infiltrate PEO). In the case of the infiltrated PE, although its crystallization temperature was also lowered by confinement within a large number of nanocylinders, its value is still at least 75 degrees C above T-g; therefore, its nucleation is probably originated at the surface of the nanocylinders and cannot be considered homogeneous bulk nucleation. Strongly segregated semicrystalline PE-b-PS diblock copolymers were infiltrated into AAO templates for the first time, thereby creating a nanostructured hybrid material where the PE phase experienced double confinement for some compositions (i.e, the phase-segregated confinement within a vitreous PS matrix and the physical confinement within the nanopores). Fractionated crystallization was observed for the PE block microdomains within both neat copolymers and infiltrated ones. However, in the case of the infiltrated block copolymers, the dominant crystallization was always that produced at maximum supercooling, as expected for the extreme confinement environment. In the cases of PE blocks that self-assemble to form 42 nm diameter cylinders and 24 nm spheres within the neat block copolymers, the PE phase crystallization was impossible to detect once they were infiltrated into the 35 nm templates, as opposed to the 60 nm case where the lowest crystallization and melting temperatures were recorded. The crystallization and melting temperatures decreased as the volume of the crystallizing phase decreased (i.e., with a decrease in template pore volume, for neat PE, or PE phase volume within neat of infiltrated copolymers). The nucleation of the confined PE block microdomains is probably originated at their surface.