Natural-abundance 13C NMR spectra of pure nylon 6 and its composites were measured with CP and MAS techniques. The microcomposites were made by adding glass microspheres, 3-aminopropyltriethoxysilane coupling agent, or prefunctionalized glass spheres. 13C NMR spectra of these samples showed the presence of three components: two cryst. phases and an amorphous component. The chem. shifts of both carbons adjacent to the amide group were sensitive to the nylon 6 crystal structure; in the dominant a polymorph, C1 and C5 were 3 ppm downfield from their chem. shift values in the g polymorph. Both the X-ray diffraction powder patterns and the 13C NMR spectra showed a change in the relative amts. of the two cryst. components when the composites were made.