On the basis of variable-temperature ultrahigh vacuum scanning tunneling microscopy data, we propose a two-dimensional phase diagram of monolayer decanethiol on Au(lll). Four triple-point temperatures were determined: T-1 at approximate to 27 degreesC, T-2 at approximate to 33 degreesC, T-3 at approximate to 35 degreesC, and T-4 at approximate to 56 degreesC. T-1 defines the lowest temperature melting point, and T-4 defines the temperature above which striped phases are metastable. These data provide a fundamental framework to understand and control mesoscale monolayer structure; moreover, they provide fundamental insight into the two-dimensional phase behavior of molecules with many degrees-of-freedom.