The mechanism for triplet-sensitized Z-E isomerizations of Z,Z-, E,Z-, and E,E-1,5-bisstyryl-3,7-dimethyl-cyclooctatetraene (ZZ, EZ, and EE) has been studied by quantum yield measurements and laser flash photolyses. ZZ isomerizes via EZ as a ground-state intermediate to EE with small quantum yields. At the photostationary state the isomer mixture consists of more than 99% EE. The three E,Z isomers have different T-1-T-n, absorption spectra. These results can be explained with a triplet energy surface, with local minima at geometries, in which the noncyclic double bonds are planar. The dominating decay process of (3)ZZ* (3)EZ*, and (3)EE*, is intersystem crossing to the corresponding ground states. Their triplet energies were estimated to be between 40 and 43 kcal mol(-1).