Unsaturated structures in polyolefin polymers are important in many respects. In this work, new vinyl and vinylidene structures were identified in poly(ethylene-co-1-octene) copolymers. The combination of careful sample selection and model compounds provided clear evidence for the assignment of these structures. More importantly, a new method was developed to differentiate and quantify for the first time terminal and internal unsaturations in ethylene-co-1-octene copolymers. The method described here will be generally applicable to many different polyolefins.