Molecular self-assembly is a key to wide-ranging nano-and micro-scale applications in numerous fields. Understanding its underlying molecular level science is therefore of prime importance. This study resolves theÅ-scale structure of the earliest and simplest self-assembled monolayer (SAM): octadecanol on amorphous-SiO 2 -terminated Si (001) substrate, and determines the structure's temperature evolution. At low temperatures lateral hexagonal order exists, with close-packed, surface-normal molecules. ∼ 12 • C above the alkanol's bulk melting a fully-reversible disordering transition occurs to a novel "stretched liquid" phase, laterally disordered, but only ∼ 15% thinner SAM than the crystalline phase. The SAM persists to ≥ 100 • C. A thermodynamic model yields here a headgroup-substrate bond energy ∼ 40% lower than on crystalline sapphire, highlighting the importance of the substrate's order, and near-epitaxy, for the SAM's ordering and stability.