It is proposed that differences between the thermal expansion of the macroscopic and crystal lattice volumes of crystalline polymers might be attributed to thermal generation of new substitutional sites. Consideration of the molecular arrangements suggests that vacancies can be generated within the lamellae at molecular folds and ends and at any point along some helical molecules. Part of the displaced molecule can be transported by point defects to the crystal surface where new sites are formed. Analysis of the available data for polyethylene shows the concentration of new sites (&Dgr;n/n) to be 2.2×10−2at 100°C with an energy of creation of 0.2 eV and an entropy of creation of 5.3×10−4eV/°K per site. It is pointed out that the phenomenon could account for the so‐called glass transitions and is, therefore, closely related to the ``amorphous'' relaxations observed in crystalline polymers. The need for additional experiments is discussed.