AbstractCrystals of poly(ε‐caprolactone) thicken at a measurable rate at the crystallization temperature. Melting points have been converted into lamellar thicknesses for three molecular weights and the lamellar thickening behavior analyzed using the Sanchez‐Eby approach of a relaxation time for thickening. Isothermal thickening curves are S shaped and can be superposed through horizontal shifts for a given molecular weight. Data for different molecular weights can be superposed using a horizontal shift combined with a small vertical shift. Shift factors have been treated using an Arrhenius approach to give an activation energy for thickening of 100 kcal/mol. Analysis using the Sanchez‐Eby approach leads to the conclusion that the active unit in thickening is approximately one stem