The host-guest properties of acetylated aryl pyrogallol[4]arene (AcPy) were studied as a Langmuir monolayer at the air-water interface. The self-assembled Langmuir monolayer properties and interactions with monovalent and divalent metal cations were investigated using surface pressure- and surface potential-area isotherms, compression-decompression cycles, stability, and infrared reflection-absorption spectroscopy (IRRAS). A favorable interaction is observed for complexation between acetylated aryl pyrogallol[4]arene with divalent calcium and cadmium cations, while no interaction is observed for monovalent sodium and potassium cations. Spectroscopic techniques allow for discrimination between cadmium and calcium complexation.