Annealed organic monolayers of dodecanethiol (CH₃(CH₂)₁₁SH) self-assembled on Au(111) have been investigated by scanning tunnelling microscopy. By using very high tunnelling resistances (TΩ) we find evidence of well-ordered molecular domains at room temperature and observe two different types of c(4 × 2) superlattice structures with the same rectangular unit cell. In contrast to theoretical calculations each unit cell contains four inequivalent molecules. Possible reasons for this discrepancy are discussed.