The monolayer properties of the amphiphile N,N'-bis(10,12-pentacosadiynyloxycarbonylmethyl)dithiooxamide at the air-water interface were studied by measuring surface pressure-area isotherms. The amphiphile itself does not form stable monolayers on an aqueous subphase but when Cu(II) ions are added to the subphase a coordination polymer is formed, which forms stable monolayers at the air-water interface. The monolayer stability is strongly influenced by the counterion, pH and temperature of the subphase. Electron microscopy shows that at a low surface pressure (about 2 mN m(-1)) a monolayer of loosely packed islands is formed. When the surface pressure has increased a more homogeneous monolayer is formed with small holes which disappear when the monolayer is stabilised at a constant surface pressure for a few hours. The monolayers of the coordination polymer can be transferred onto solid substrates by a Y-type transfer. The presence of Cu(II) ions is confirmed by XPS measurements and UV/Vis spectroscopy shows a strong absorption at 370 nm, indicating that a strongly conjugated coordination polymer is formed at the air-water interface. SAXR measurements reveal that the LB films have a rather irregular layer structure and upon exposure to UV light, the diacetylene groups are only partly converted to polydiacetylene; moreover, the coordination polymer is partly destroyed.