The NMR spin-diffusion process taking place in a morphology represented by three different domains with arbitrary sizes and diffusivities is investigated. General analytical solutions valid for the full range of spin-diffusion times were obtained for a one-dimensional lamellar morphology. The accuracy of these solutions has been tested by predicting the domain sizes for a poly(styrene-b-methyphenylsiloxane) diblock copolymer and semicrystalline poly(ethylene oxide) and comparing them with previously reported data. The effects of large changes in the size of the interface domain and the spin-diffusion coefficients of this domain on the spin-diffusion decay and build-up curves can be analyzed by numerical simulations. General analytical solutions of the spin-diffusion equations were used for investigating the complex morphology of high-speed melt-spun nylon 6 fibers. The NMR mobile amorphous, NMR less-mobile amorphous, and crystalline phases of nylon 6 fibers hydrated with DO were detected and quantified using H-1 spectra and spin-lattice relaxation rates. Proton spin-diffusion experiments were performed on nylon 6 fibers using a dipolar filter based on magic and polarization-echoes for the mobile amorphous phase. The results of this experiment can be interpreted by considering that the H-1 magnetization front which emanates from the mobile amorphous phase explores crystalline/less-mobile amorphous aggregates. The domain sizes of the mobile amorphous phase, the interface, and aggregates can be estimated using analytical solutions of the spin-diffusion equations and correlated with the spinning speed and the draw ratio of the nylon 6 fibers.