In contrast to monolayers and Langmuir-Blodgett (LB) films of divalent metal salts of fatty acids, these films containing trivalent metal ions have not been studied enough yet. This work deals with the study of monolayers and LB films of yttrium stearate. The dependencies of monolayer and LB film compositions on subphase pH were obtained from pressure-area isotherms and Fourier transform infrared spectroscopy (FTIR) spectra. Two reaction mechanisms of an acid conversion into the stearate on the water surface were identified. The mechanisms are determined by pH and metal ion concentration in the subphase. High quality LB films of yttrium stearate (up to 30 monolayers) were deposited on various solid substrates. During LB films preparation, monolayer masses were measured using a quartz crystal microbalance. In a comparison of those with theoretically calculated values, the monolayer composition was identified. At high pH, the formation of basic yttrium salt was found. The results obtained are compared with the literature data for the system being studied.