We have used grazing incidence X-ray diffraction and X-ray specular reflectivity to study Langmuir films of a phenylacetylene macrocycle (PAM), a ring-shaped molecule known to form a tubular liquid crystal. PAM adopts an "edge-on" molecular arrangement at the air-water interface. The local structure is quite similar to that of the corresponding bulk columnar liquid crystal, but with enhanced order in the intracolumnar direction. The columnar order is disrupted by CsCl in the subphase and strongly enhanced by KCl in the subphase.