We describe a comparative study of long time isothermal melting kinetics of Ziegler-Natta catalyzed, and metallocene catalyzed, isotactic poly(propylenes). The role of the transverse lamellae in stabilizing the crystalline structures against melting is presented in light of the different molecular chain architecture and lamellar structure of these poly(propylenes). Both poly(propylenes) were first isothermally crystallized at 152 degreesC. Real time in-situ wide- and small-angle X-ray scattering (WAXS and SAXS) were used to study isothermal melting kinetics in a temperature region between melting of transverse and dominant (radial) lamellae, i.e., when most transverse lamellae are molten. Changes in WAYS crystalline peak height, SAXS invariant, and Bragg peak height with time all confirm that Ziegler-Natta iPP had slower melting kinetics than metallocene iPP. Melting kinetics is, apparently, unique to poly(propylene) as a consequence of a favorable nearly orthogonal epitaxial crystallization. What is observed is the instability of the dominant thicker lamellae due to the removal of the epitaxy. Both Ziegler-Natta and metallocene catalyzed polymers show a prolonged, gradual decay of crystallinity with time. Hence, melting kinetics is a general phenomenon observed in poly(propylenes) that crystallize preferentially in the alpha (monoclinic) modification. The crystallization conditions under which these iPPs exhibit melting kinetics are when crystallization occurs at relatively low undercooling, when the crystallites are not subject to reorganization.