2D phase transition of PF6 adlayers at the electrified ionic liquid/Au(111) interface has been investigated by in situ STM. At the potential above -0.2 V vs Pt reference, PF6 anions form Moire-like patterns on Au(111). When the potential is changed towards more negative values, the structural changes occur gradually, and a (root 3 x root 3) phase forms at approximate -0.45 V. More interestingly, the Moire-like patterns are found to coexist with the uncompressed structures in most cases. The formation of the ordered adlayers can be attributed to the different solvation of PF6 in ionic liquid in comparison with aqueous solution. (c) 2006 Elsevier B.V. All rights reserved.