We present evidence for the existence of ordered tilted-chain phases of self-assembled monolayers of C18H37SH alkylthiols (C18) on Au(111). Tilted phases have been observed before under compressive stress in thiol and silanes monolayers. In the present experiments with C18, tilted phases have been observed to exist also in the absence of applied pressure. Atomic force microscopy (AFM) has been used to determine the thickness and lattice periodicity of the films. Two phases have been found to coexist: a hexagonal one with (root 3x root3)R30 degrees periodicity (tilt angle 35 degrees), which is characteristic of complete monolayers, and a less dense one with (2x root3)rect periodicity. This corresponds to a 50 degrees tilted phase, predicted by a chain-interlocking model. The new phase can be irreversibly transformed into the hexagonal phase by mechanical perturbation with the AFM tip. (C) 2001 American Institute of Physics.