Copolymeric poly(2-vinylimidazoline), comprising the ring-closed 2-imidazoline and ring-opened linear amide repeat units, was characterized by C-13-NMR spectroscopy. This was facilitated by the C-13-NMR analyses of the model compound forms of the copolymer’s repeat units, 2-ethylimidazoline and N-propionylethylenediamine. Quantitative determinations were made for the percentages of ring-opened, linear amide, repeat units and residual ethylenediamine present in different batches of the poly(2-vinylimidazoline) copolymer.