Titanium‐based Ziegler–Natta catalysts for the polymerization of buta‐1,3‐diene give linear trimers of butadiene among other by‐products. A combination of ¹H and ¹³C NMR, DEPT and 2D‐INADEQUATE gives exhaustive assignments of their ¹³C NMR spectra and affords their structures. All these trimers are dodeca‐2,4,6,10‐tetraenes which have an isolated cis double bond. The internal double bond of the conjugated triad has a trans configuration in each isomer, and the trimers differ only in the outer double bonds. The ¹³C NMR spectra of the timers reveal that the signal position of the carbon involved in the conjugated double bond series is mainly controlled by the configuration of the adjacent double bond and, to a lesser extent, by the geometry of its own double bond. The oligomers studied serve as models for polyacetylene. Some reassignments in the spectra of oligomers of terminal acetylenes are proposed.