Although 2-[2-(2-pyridyl) ethenyl] indole undergoes one-way E → Z isomerization in the excited singlet state, it isomerizes between the E and Z isomers in the excited triplet state. On the basis of the determination of the quantum yields of isomerization and the photostationary state isomer ratios, as well as transient spectroscopic measurements, the potential energy surfaces for E- Z isomerization in the triplet state have been proposed. Specific rotamers of the E and Z isomers are found to be responsible for the triplet state isomerization reactions.