Structure evolution during isothermal crystallization of isotactic polypropylene and the changes during a subsequent heating to the melt were analyzed by time- and temperature-dependent small-angle X-ray scattering experiments. Results demonstrate that isotactic polypropylene, when forming the a-polymorph, is governed by the same general laws for crystallization and melting as syndiotactic polypropylene. Crystal thicknesses are inversely proportional to the supercooling under a characteristic temperature T-c(infinity) which is located above the equilibrium melting point of a sample. The crystallization line describing this dependence can be understood as representing the stability limit of a well-defined initial state with higher free energy. Its transformation into the final lamellar two-phase structure provides the stabilization expressed in the difference between the crystallization and melting temperature. Observations and data from other sources can be seen as indicating that (i) crystal thicknesses are independent of the isotacticity and that (ii) the initial state is composed of crystal blocks in planar assemblies, in agreement with corresponding observations on syndiotactic polypropylene. The small-angle X-ray scattering experiments provide a detailed insight into the structure changes during the continuous melting. For high crystallization temperatures no recrystallization occurs. Crystals with different stabilities, all having the same thickness, melt consecutively. Structures produced at low crystallization temperatures are less stable. Here heating leads to repeated melting-recrystallization processes associated with jumplike changes in the length scale of the structure.