1,3,5-Hydroxy-triazo-benzene (H-TAB) was synthesized through a coupling-oxidation protocol. Temperature-controlled UV, IR, and ab initio calculation were carried out to investigate the cis-trans thermal isomerization of H-TAB. In temperature-controlled UV experiments, lambda(max) of the pi-pi* band and for the trans conformation at 335 nm and that for the cis form at 282 nm are shifted by increased temperature; band intensities of the pi-pi* transition decrease and of the pi-pi* band is shifted toward the high-energy region. The maximum peak at 2922 cm(-1) is shifted to 2926 cm-1, and that at 2852 cm-1 is shifted to 2856 cm(-1) at increased temperature in the temperature-controlled IR experiment. Ab initio calculation reveals that the cis conformation of H-TAB is more stable than the trans form because the cis form has less spatial repulsion. Therefore, the ground-state energy difference induced by steric repulsion of the benzene unit is the driving force of the blue shift in the thermal IR and UV spectra for the triazo-benzene. (C) 2006 Elsevier B.V. All rights reserved.