This paper presents the first application of a suite of three-dimensional (3D) NMR techniques for the characterization of complex microstructures in hydrocarbon-based terpolymers. The 3D-NMR experiments are used to disperse and assign the H-1 and C-13 resonances of C-13-labeled poly(ethylene-co-1,2,3-C-13(3)-n-butyl acrylate-co-carbon monoxide). The one-dimensional (1D) and two-dimensional 2D-NMR spectra of the complex mixture of structures in this polymer are far too complex to interpret. Selective C-13-labeling of all or one carbon in a monomer and use of shaped pulses (which treat the resonances from the labeled C-13 as if they were from a third NMR active nucleus) along with 3D-NMR is a solution to this complex problem as it greatly simplifies the spectra by dispersing the resonances into a third dimension and provides atomic connectivity information among three different nuclei. This paper represents the first demonstration of new 3D-NMR methodology for determining structures and resonance assignments of complex hydrocarbon-based structures and mixtures.