Highly ordered monolayers of a long‐chain molecule, dotriacontane (C32H66), have been obtained by self‐assembly at the interface between an organic solution and the basal planes of two semiconductors, MoSe2 and MoS2. Scanning tunneling microscopy in situ at the solid–fluid interfaces revealed that the adsorbate lattices are close packed and oriented relative to the substrate lattices, but that they are not simply commensurate. The results indicate that for flexible chain molecules, which interact only weakly with the surface of a solid substrate, a high degree of order in the adsorbed monolayers is induced by the atomical flatness of the substrate, while the coincidence of lattice parameters and the specific surface chemistry play only a minor role. Details of the packing, including the symmetry of the adsorbate unit cell, depend on the particular substrate. The results imply that atomical flatness is a key factor for the fabrication of highly ordered organic/inorganic heterostructures.