Crystalline monolayers of three aldehydes with an odd number of carbon atoms in the alkyl chain (C₇, C₉ and C₁₁) at low coverages are observed by a combination of X-ray and neutron diffraction. Analysis of the diffraction data is discussed and possible monolayer crystal structures are proposed; although unique structures could not be ascertained for all molecules. We conclude that the structures are flat on the surface, with the molecules lying in the plane of the layer. The C₁₁ homologue is determined to have a plane group of either p2, pgb or pgg, and for the C₇ homologue the p2 plane group is preferred.