Langmuir-Blodgett (LB) films of cobalt octadecylphosphonate were prepared and shown to form mixed organic/inorganic layered films of the formula Co(O3PC18H37). H2O possessing two-dimensional continuous solid metal phosphonate layers. The films were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ellipsometry, and attenuated total reflectance Fourier transform infrared (FTIR) spectroscopy, and they were shown to be LB analogs of the known solid-state layered cobalt phosphonates. Reactivity seen in the solid-state cobalt phosphates was also demonstrated for the LB films. Like the solid-state analogs, the cobalt phosphonate LB films can be dehydrated and reacted to uptake ammonia from the vapor phase.