High-resolution carbon-13 NMR of bibullvalenyl solutions and magic angle spinning (MAS) of solid bibullvalenyl are reported over a wide temperature range and interpreted in terms of the isomeric distribution and the kinetics of the bond shift (Cope) rearrangements. In solution, bibullvalenyl exists predominantly as a mixture of the 3-3, 3-2, and 2-2 isomers, in which the bullvalenyl radicals are linked at the olefinic carbons. Their relative concentrations in the temperature range -50 to -20 degrees C are 0.67, 0.28, and 0.05, respectively. Above -30 degrees C bond shift rearrangement results in broadening of the NMR signals. Detailed analysis of these dynamic spectra indicates the occurence of at least three main rearrangement processes : (i) direct interconversion of the isomeric pairs 3-X reversible arrow 2-X (X = 2 or 3), (ii) degenerate rearrangement of the 2-X isomers, 2-X reversible arrow 2*-X (where the asterisk indicates a rearranged bullvalenyl radical), and (iii) a pseudodegenerate rearrangement of the 3-X isomers via the intermediate 1-X, 3-X --> [1-X] --> 3*-X, and possibly also via the intermediate 4-X. In the solid state, bibullvalenyl crystallizes entirely as isomer 3-3, with the two bullvalenyl radicals most probably in crystallographically unrelated sites. Above room temperature the MAS spectra exhibit selective line broadening due to the Cope rearrangement. Detailed analysis shows that the pathway for the reaction involves the isomer 1-3, which serves as a transient intermediate in the reaction, It is likely that at higher temperatures additional pathways involving the isomers 4-3 and/or 2-3 also contribute to the line broadening in the solid state.