The double melting endotherm of spunbonded isotactic polypropylene (IPP) fabrics was investigated by monitoring changes in the solid-state NMR spectrum that result from thermal annealing. The DSC melting thermogram was found to change from a double to a single endotherm at anneal temperatures greater than or equal to 156 degrees C, with a concomitant increase in percent crystallinity. All of the carbon resonances in the CP/MAS NMR spectrum of the purely crystalline phase of IPP were found to be composed of multiple peaks with relative intensities that depend on anneal temperature. By monitoring the changes in the distribution of intensity among the various peaks of a given resonance, a transition temperature of 156 degrees C was identified. Arguments are presented that this redistribution of intensity within a given carbon resonance characterizes the transformation from the cu, to the or, monoclinic crystal form. The exothermicity associated with this transformation is responsible for the observation of a double melting endotherm by DSC. The splitting patterns observed in the NMR spectrum are discussed in terms of interlayer distances between layers of isochiral helices and the density of exposed methyls at the contact faces of these interlayers.