This Letter presents a study on the oxidation of electrochemically deposited Sn monolayer on Au(1 1 1) surface and STM tip-induced reduction of as-prepared ultra thin SnO film. A threshold bias of 0.6 V (tip negative) at a low tunneling current of similar to50 pA is required to image the as-formed SnO thin film by STM, typical of a semiconductor characteristic. Increasing the tunneling current to similar to2 nA leads to the reduction of the SnO back to Sn. Based on the energy level calculation for the SnO, a mechanism involving direct electron tunneling is proposed to account for the tip-induced reduction. (C) 2003 Elsevier Science B.V. All rights reserved.