Insoluble alkanes have been observed by electrochemical scanning tunneling microscopy (STM) to form ordered monolayers at the Au(111)/0.1 M HClO4 solution interface. Hexadecane molecules self-assemble into ordered layers that are stable over the potential range from 0.15 V-SCE to 0.55 V-SCE, on both reconstructed and unreconstructed Au(111) surfaces under electrochemical control. The hexadecane molecules appear as 2.2 nm long and 0.45 nm wide rods, suggesting an extended conformation. STM images show that a reversible order-disorder transition can be induced by moving the electrode potential positive or negative of the stable potential region (0.15 V-SCE to 0.55 VSCE). The hexadecane molecules aggregate immediately after lifting of the surface reconstruction by a positive potential step to 0.65 V-SCE. Reversible disappearance of the ordered alkane structure is also observed when the potential is stepped below 0.15 V-SCE. The molecules resume an ordered lamellar structure upon return of the substrate to the stable potential region. A critical role for the aqueous solvent in inducing the phase transition is proposed.