Self-assembled monolayers (SAMs) of terphenylthiol (TPT) on gold Au(111) substrates exhibit well-ordered structures over large scales if they are annealed in ethanol at 40 degrees C after their formation. Using high-resolution STM, two distinct, ordered phases could be observed. The simpler phase, designated as alpha-phase, consists of closely packed molecules in the well-known (2 root 3 x root 3)R30 degrees structure. It could be demonstrated that under less suitable imaging conditions this phase can be mistaken as the hexagonal (root 3 x root 3)R30 degrees, which resolves a discrepancy in between previous reports. The second phase is characterized 6 by a stripe pattern with a periodicity of 2.0 nm and can be described by a point-on-line incommensurate (4 x n) ([40 0n]) lattice with n close to 8. This beta-phase contains four pairs of terphenylthiolate molecules, which might be held together by either disulfide bonds or the recently discussed S-Au-S motif; and is thus 35% +/- 15% less densely packed than the alpha-phase. The coexistence of these phases explains the variability of spectroscopic results obtained in the past for terphenylthiolate layers, since their relative proportion determines the average thicknesses/tilt angles found in these studies.