In the interaction of iodine atoms with butene-2 at 65 °C no addition products are observed although cis–trans isomerization of the butene occurs readily. Iodine atoms have been generated by photolyzing iodine in three spectral regions. The limiting quantum yield of the isomerization reaction is about 0.1 at 4358 Å. The low quantum yield shows that chain reactions, similar to those observed in olefin photochlorinations and photobrominations, do not take place. A mechanism involving initial formation of π-complexes between iodine atoms and butene-2 is tentatively considered.