The crystalline structure of hydroxy-functionalized hexanethiol self-assembled monolayers on Au(111) were characterized using gas-phase transport of molecular precursors and an ultrahigh vacuum scanning tunneling microscope. In the pristine state the monolayer exhibits a commensurate lattice with an oblique primitive unit-cell of dimensions a=3, b=root 13, alpha=a tan(2 root 3); a structure that is fundamentally different than the hexagonal lattice observed for methyl-terminated monolayers. Hydration of the monolayer results in conversion to a polymorphic phase. These results demonstrate the importance of end-group chemical bonding in the molecular packing of this widely studied class of materials.