Following the constant pressure relaxation on different subphases and the deposition on solid substrates, the structure and morphology of arachidic acid monolayers were studied by X-ray reflectometry and atomic force microscopy. The growth of three-dimensional (3D) phase nuclei was observed from the first minutes of the relaxation process in a monolayer relaxed on a subphase of low pH less than or equal to 3 with or without metal ions. With an increase in the relaxation time, the granule-like nuclei were transformed into platelike multilayer islands. The structure of 3D islands was commensurate with the B and C forms of bulk arachidic acid, with the B phase being prevalent after a long period of relaxation. During the storage of the samples in air, the granule-like islands were unstable and, after 2 months, transformed into the more stable platelike islands. The relaxation process was much slower in the case of binding the monolayer to cadmium or lead cations in a subphase of pH 5.3. After about 1 h of relaxation, only low folds arrayed with a hexagonal symmetry were observed on the monolayer of cadmium arachidate. On monolayers of lead arachidate, 1-2 bilayer islands emerged with hexagonal boundaries after a relaxation of some 3 h.