Compounds which undergo fast and reversible valence-bond isomerization (i.e. which have fluctuating cyclopropyl and/or double bonds) are taken to be those in which the average life-time of the valence-bond isomers at 0 °C is of the order of 100 seconds at most, and for which the maximum activation energy of the bond migration is 20 kcal/mole. The fast, reversible bond shift can scarcely be detected by chemical means, but can be recognized by NMR spectroscopy, since the time for which a proton occupies a position with a given magnetic environment has a pronounced influence on the NMR spectrum. Some examples of molecules with fluctuating bonds are cyclooctatetraene and its derivatives, unsaturated seven- membered ring systems, homotropilidene, bridged homotropilidene systems, bullvalene, and substituted bullvalenes. The last class is particularly interesting, since here the carbon atoms are continually changing their relative positions and neighbors.