AbstractThe half‐life (τ1/2) for thermal isomerization of cis‐4‐hydroxyazobenzene (c‐HOAB) to its trans isomer is highly dependent on the solvent used. For non‐aromatic solvents, the cis isomer is extremely unstable at 293 K (τ1/2 < 1 min in methanol and acetonitrile and 6 min in cyclohexane), whereas the cis isomer is surprisingly stable in benzene (τ1/2 = 125 min). However, addition of hydrogen chloride and triethylamine in benzene causes a remarkable decrease in τ1/2. On the basis of ab initio molecular orbital calculations of the binding energies and the optimum structures of c‐HOAB–solvent complexes, a mechanism for the thermal cis‐to‐trans isomerization reaction involving hydrogen‐bonded dimers and complexes is suggested. Copyright © 2005 John Wiley & Sons, Ltd.