In-situ electrochemical scanning tunnelling microscopy (STM) was applied to the investigation of the adsorption of iodine (I) on a well-defined Ni(111) single crystal electrode in an aqueous solution of 10 mM HF + 1 mM KI under electrochemical potential control. The (root 3 x root 3)R30 degrees adlattice of I was observed by in-situ STM and confirmed by low-energy electron diffraction (LEED) using an ultrahigh vacuum-electrochemical apparatus. Adsorption of I took place in the potential region of the second anodic peak, and the I adlayer remained on the surface even with the anodic dissolution in progress. The I adatoms were desorbed upon sweeping the potential into the negative region, in which the Ni(111)-(1 x 1) structure was discerned clearly by in-situ STM. The anodic dissolution of Ni(111) with the adlayer of I was found to proceed by the layer-by-layer mode. Reactivity of the steps is discussed based on the geometrical configuration of Ni and I atoms near the step edges.