It was commonly accepted the alkanethiols (C₂H_2n+1SH) self assemble on Au(111) into the (root 3 x root 3)R30 degrees structure by chemisorption of the sulfur atoms to the three-fold hollow sites. However, the observation of (p x root 3) structures and evidence for dimerized sulfurs on the substrate have questioned this model. We have imaged alkanethiols on Au(111) for 4 less than or equal to n less than or equal to 8 by scanning tunneling microscopy under ambient conditions. We found (p x root 3) structures for n less than or equal to 6 and the (root 3 x root 3)R30 degrees structure for n greater than or equal to 7 both for high densities of (22 +/- 2)Angstrom(2) per molecule indicating a dependence of structure on chain-length. We can describe our observations by two distinct crystalline phases of dimerized thiol molecules.