Surface-specific X-ray methods and surface tensiometry are used to study the structure of Langmuir films of alkyl diols with 15 ≤ n ≤ 24 carbons supported on a liquid mercury subphase. Phases of either mono- or bi-layers of surface-parallel molecules are found, depending on surface pressure. Although both ends of the diol molecule can form hydrogen bonds, as do corresponding-length α,ω-alkyldiacids, the consequent 2D order occurring in diacids is not observed here. Instead, one-dimensional order along the chain-normal direction is established for the longer diols, exhibiting two length scales, corresponding both to the molecular width, and differing by ∼3%. The reasons for, and molecular interactions leading to, this structure are discussed.