The correct value of the equilibrium melting point of isotactic polypropylene has been determined using small-angle X-ray diffraction. The conflict in the literature between the two very different values obtained through extrapolation of melting point versus crystallization temperature data has been resolved. It is demonstrated through studies of the melting point of polypropylene as a function of crystallization time that the dependence of melting point elevation on supercooling is the opposite of that of polyethylene. The thickening process is shown to be most effective at low supercoolings, leading to abnormally high melting points for specimens crystallized at low supercoolings. The equilibrium melting point of isotactic polypropylene is close to 186-degrees-C. It is believed that the observed behavior is a direct result of polypropylene crystallizing in regimes II and III, unlike bulk linear polyethylene, which crystallizes in regimes I and II. It is suggested that the behavior may be directly related to the length of continuous adjacent reentry folding generated under the different regimes.