Thermal analyses were performed for determining the equilibrium melting temperatures T-m(0) of the respective alpha- and beta-crystal in melt-crystallized polymorphic poly(heptamethylene terephthalate) (PHepT) using both linear and nonlinear Hoffman-Weeks (H-W) methods for comparison of validity. These two crystals in PHepT do not differ much in their melting temperatures. The equilibrium melting temperatures of the alpha- and beta-crystal as determined by the linear H-W method are 98 degrees C and 100.1 degrees C, respectively; but the nonlinear H-W method yielded higher values for both crystals. The equilibrium melting temperatures of the alpha- and beta-crystal according to the nonlinear H-W method are 121 degrees C and 122.5 degrees C, respectively. Both methods consistently indicate that T-m(0) of the beta-crystal is only slightly higher than that of the alpha-crystal. Such small difference in T-m(0) between the alpha- and the beta-crystal causes difficulties in judging the relative thermodynamic stability of these two crystals. Thus, kinetics of these two crystals was compared using the Avrami and Ozawa theory. The crystallization produced by quenching from T-max = 110 degrees C and 150 degrees C shows a heterogeneous and homogeneous nucleation mechanism, respectively. The lower T-max = 110 degrees C leads to heterogeneous nucleation and only alpha-crystal in PHepT, whose crystallization rates at same T-c are much higher than crystallization rates by quenching from T-max = 150 degrees C leading to either alpha- or beta-crystal with homogeneous nucleation. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1839-1851, 2009