Preferential diffusion of deuterated solvents into the amorphous regions of a semicrystalline polymer enhances the contrast between the crystalline and amorphous regions measurable by small-angle neutron scattering. This scattering in nylons from the diffusion of D2O and deuterated ethylene glycol (d-EG) is analyzed by identifying the distinct contribution to scattering From the two amorphous regions, one in the interlamellar spaces and the other outside the lamellar stacks. The central diffuse scattering (I-d) is the non-Bragg, liquidlike, or independent scattering, and is attributed to the solvents (D2O/d-EG) in the amorphous domains outside the lamellar stacks. The lamellar scattering (I-l) is the interference peak from the lamellae in the stacks and is used to evaluate the distance between the lamellae, the thickness of the interlamellar spaces, and the coherence length of the lamellar stacks. The invariant calculations show that 70%-80% of the lamellar stack is crystalline. About one-third of the amorphous material in a highly crystalline nylon is in the interlamellar space, and two-thirds is outside the lamellar stacks. The thickness of the interlamellar amorphous regions into which solvent molecules diffuse varies from 10 to 60 Angstrom depending on the thermal history and is a major contributor to the observed increase in lamellae spacing. Structural changes in nylon 6 immersed in water are accelerated at 125 degrees C, and this temperature could be the hydrated-equivalent of the Brill transition observed at 160 degrees C in dry nylon 6. Water or EG diffuses into the fold surfaces of nylon lamellae at elevated temperatures, and subsequent structural changes are accompanied by hydrolysis of the nylon chains. EG being a stronger solvent reduces the lamellar thickness at elevated temperatures.