Several lanthanide(III) octadecylphosphonate Langmuir−Blodgett (LB) films have been prepared and characterized. Films with La3+, Ce3+, Sm3+, and Gd3+ are formed one bilayer at a time using a Y-type deposition procedure that allows the metal phosphonate continuous lattice network to crystallize during the upstroke of the film transfer. Comparison of the phosphonate IR stretching modes of the LB films to those of known model solids shows that each film is an LB analogue of the powdered solid-state lanthanide butylphosphonates of formula LnIIIH(O3PC4H9)2 that possess a two-dimensional ionic covalent metal phosphonate network. X-ray photoelectron spectroscopy (XPS) shows that the P/Ln ratio in each of the transferred films is 2:1, which corresponds to the stoichiometry in the known layered materials, and X-ray diffraction (XRD) shows that the films are layered each with a bilayer thickness of 51 ± 1 Å. The behavior of octadecylphosphonic acid Langmuir monolayers on subphases containing each metal ion is studied as a function of pH, and general procedures for the deposition of metal phosphonate continuous lattice LB films are developed.