Self-assembled monolayers formed by adsorption of 4-methyl-4'-mercaptobiphenyl, CH3-(C6H4)(2)-SH (BPT), on Au(111) have been studied using scanning tunneling microscopy. The results show that with increasing coverage (resulting from longer immersion times) BPT forms a series of different structural phases with different molecular arrangements, closely resembling the behavior reported previously for n-alkanethiolate adlayers. For short immersion times, striped structures are observed (alpha and beta), where the molecules are orientated with their axes parallel to the surface. Longer immersion times yield additional phases, namely, an ordered chi-phase, where the molecules are proposed to be oriented with their molecular axis tilted away from the surface, a disordered delta phase, and, finally, a densely packed (2root3 x root3) epsilon phase where the molecular axes are orientated almost upright. Unexpectedly, for BPT after adsorption small islands are seen on the Au substrate instead of the etch pits commonly observed after formation of organothiolate adlayers.