Two samples of isotactic polypropylene (PP), with different treatment histories, were studied by carrying out a number of proton n.m.r. measurements, namely free induction decay (FID) and spin-lattice relaxation in both the laboratory and rotating frames. The results were subjected to detailed mathematical analysis, and are shown to yield information about the domain structure and intrinsic relaxation parameters. Inversion-nulled FID analysis shows that the samples contain a spatially resolved distribution of crystallinities, which is responsible for bi-exponential spin-lattice relaxation.