An infrared laser induced cis<---->trans isomerization has been investigated for 2-naphthol and its hydrogen(H)-bonded cluster with NH3 in the S-1 electronic state by UV-IR double resonance excitation in a supersonic jet. A specific isomer was pumped to a X-H (X=O, C or N) stretching vibration in S-1 by a sequential UV-IR two-photon excitation, and the resultant product species were analyzed by dispersing their fluorescence. It was found that the IR excitation induces the isomerization for the H-bonded cluster with the excitation less than 3000 cm(-1), while no isomerization was found to occur for bare 2NpOH even with an input energy of 3610 cm(-1). The substantial reduction of the isomerization barrier height represents evidence of the catalytic nature of H-bonding in the conformational isomerization. It was also found that the isomerization efficiency is very high at low IR frequency and exhibits a marked IR frequency dependence. (C) 2003 American Institute of Physics.