In situ scanning tunneling microscopy and cyclic voltammetry studies of self-assembled octyl-triazatriangulenium monolayers on Au(111) electrode surfaces in 0.1 M HClO4 reveal a complex surface phase behavior, involving two fast, highly reversible transitions between different ordered adlayer phases: With decreasing potential, the preadsorbed (root 19 x root 19)R23.4 degrees adlayer first is converted into a (7 root 3 x 7 root 3) and then into a (2 root 3 x 2 root 3)R30 degrees phase, corresponding to a stepwise increase in the local packing density of the molecules. The (7 root 3 x 7 root 3) -> (2 root 3 x 2 root 3)R30 degrees transition is accompanied by a reorientation of the peripheral octyl chains from a more planar to a close-packed vertical arrangement. This reversible potential-induced switching between a homogeneous adlayer of small vertical extension and a Au surface partially covered by islands of a compact hydrocarbon layer is attributed to changes in the adsorbate charge state and associated changes in the intermolecular interactions.