C-13 NMR progressive saturation measurements were used to investigate longitudinal relaxation in the crystal phase of semicrystalline alkanes. Novel monodisperse, ultralong n-alkanes were melt crystallized in such a way that they comprised crystals containing extended chains or once-folded chains. The crystalline morphology was confirmed using DSC, SAXS, and WAXS. Quantifying the longitudinal relaxation is critically dependent on an accurate measure of the crystallinity of the sample. Crystallinities were measured using X-ray scattering, calorimetric, and H-1 broadline NMR techniques. The longitudinal relaxation is interpreted via a solid-state chain diffusion process. Very high crystallinity was shown to suppress the diffusion. A one-dimensional diffusion model with a single reptation time was shown to represent successfully the relaxation of crystals comprising extended chains. The addition of chain folds was shown to result in a decrease in the diffusion coefficient, although the addition of branches at the fold surface did not represent a further constraint to diffusion. A dual reptation time diffusion model was developed and shown to describe successfully the relaxations of the crystals comprising folded chains. The diffusion coefficients were consistent with those previously measured for polyethylene, as was an estimate of the thickness of the interfacial region yielded by the diffusion model. The results were also analyzed using a conventional, multicomponent T-1 model, which suggested the crystalline T-1 may make a significant contribution at longer times. This explained the 100% relaxation seen for some samples.