Temperature-reversible and -irreversible morphological events could be separated in the case of linear polyethylene during quasi-isothermal crystallization by using simultaneous temperature-modulated synchrotron SAXS and WAXD. Crystallization and subsequent annealing was followed at 126 degreesC for 90 min while applying a temperature modulation with an amplitude of 1 degreesC and a period of 2 min. The crystal growth rate associated with the irreversible part of the crystallization decreases with increasing temperature in a cycle, The crystalline lamellae irreversibly thicken with time. The actual crystallite thickness, however, exhibits a superimposed modulation out of phase with that of the temperature modulation. Melting was studied during heating at 1 degreesC/min after cooling at 10 degreesC/min. A temperature modulation was superimposed with an amplitude of 2 degreesC and a period of 3 min. Once again temperature-reversible crystal thickness changes and irreversible crystal thickening could be observed.