AbstractMultiple phase transitions of polybutene‐1 in the form III modification have recently been the subject of discussion by a number of authors. The exact nature of the observed phenomena has remained in doubt, leading to the proposal of divergent theories. Differential thermal analyses and x‐ray diffraction studies of polybutene‐1 in the form III modifications have shown that the nature of the phase transformations is a function of the heating rate. At very slow heating rates (1°C./day), an apparent form III to form I' crystal‐crystal transformation, believed to be related to a thermodynamic “selection” process, has been observed. At faster heating rates (0.2–0.3°C./min.), only part of the form III modification is able to undergo the transformation to form I' the remainder undergoes a melting transition at 96°C, followed by immediate recrystallization to form II. The form I' generated in this process melts at 100°C, and also recrystallizes to the form II modification. At very fast heating rates (20–50°C./min.), form III melts at 96°C, failing to recrystallize.