The adsorption of sub-phthalocyanine molecules on Ag(111) has been studied by means of scanning tunneling microscopy (STM). The molecules are observed in different two-dimensional (2D) phases of adsorption which coexist in thermodynamic equilibrium. In the condensed phase the molecules form well-ordered islands with a honeycomb pattern. In the gas phase single molecules can be discriminated in single scan lines by characteristic tip excursions which occur randomly. The energy barrier for surface diffusion as well as the condensation energy to form 2D islands is estimated and discussed.