The isothermal crystallization of branched polyethylene (BPE), when it takes place in the presence of linear polyethylene (LPE) lamellae was found to occur above the highest crystallization temperature of the pure BPE. A standard state formed by isothermally crystallized LPE lamellae at 122 degrees C was used to provide surfaces for the crystallizing BPE at various seeding temperatures (117, 118, 119 or 120 degrees C). Isothermal treatments were carried out in a thermostatic oil bath, and evidence on the progress of crystallization of BPE was obtained by differential scanning calorimetry and transmission electron microscopy after quenching from the seeding temperature.