Tunneling-current-induced reaction in self-assembled monolayers of mercaptobutanol (HS(CH2)(4)OH:C4OH) has been investigated using ultra-high vacuum scanning tunneling microscopy (STM). Two different reaction channels (decomposition and translation of molecules) were observed depending on the coverage of C4OH. The threshold tunneling bias voltages of the two reaction channels are the same, +/-2.5 V. The possible channels for decomposition and translation of molecules are the rupture of the C-S bonding and the local heating of the ensemble of C4OH, respectively. (C) 2004 Elsevier B.V. All rights reserved.