The energetics and dynamics of the various phases of decanethiolate self-assembled monolayers on Au(111) surfaces were studied with scanning tunneling microscopy. We have observed five different phases of the decanethiolate monolayer on Au(111): four ordered phases (beta, delta, chi*, and phi) and one disordered phase (epsilon). We have determined the boundary free energies between the disordered and order phases by analyzing the thermally induced meandering of the domain boundaries. On the basis of these results, we are able to accurately predict the two-dimensional phase diagram of the decanethiolate/Au(111) system. The order-disorder phase transition of the chi* phase occurs at 295 K, followed by the order-disorder phase transition of the beta phase at 325 K. Above temperatures of 325 K, only the densely packed phi and disordered epsilon phases remain. Our findings are in good agreement with the phase diagram of the decanethiolate/Au(111) system that was put forward by Poirier et al. [Langmuir 2001,17 (4), 1176-1183].