The title compound (10) was prepared, along with the 4-cyano isomer (11), by the direct irradiation of 5-cyanobenzocyclooctatetraene (9); the yields were 82% (Φ10 = 0.0030) and 12% (Φ11 = 0.0004) respectively. Triene10was thermally and photochemically reactive. Heating solutions of10at 150 °C for 1 h gave COT9quantitatively. On direct irradiation10forms 5-cyanobenzosemibullvalene (12; 5%, Φ = 0.019), COT9(70%, Φ = 0.56), and 1-cyanonaphthalene (14%, Φ = 0.078). Sensitized irradiation of10gave9exclusively (92%, Φ = 0.88). COT9was also produced by the direct irradiation of semibullvalene12(75%, Φ = 0.13). Studies with deuterium labelled10suggest that the photoformation of COT9involves a simple electrocyclic opening of the cyclobutene ring of the triene. Additionally, the labelling results indicate that the formation of semibullvalene12from10derives from the operation of two reaction pathways, the major one of which appears to be a Zimmerman di-π-methane rearrangement. The mechanism proposed for the minor pathway to12has not been observed in other bicyclo[4.2.0]octatrienes.Keywords: mechanisms, rearrangements, photochemistry, di-π-methane.