Monolayers of arachidic acid, octadecyldimethylamine oxide (C(18)DAO), and their mixtures were prepared at a surface pressure of 25 mN m(-1) on a mica substrate by the Langmuir-Blodgett method and were observed by use of an atomic force microscope (AFM). Arachidic acid molecules in monolayer arranged with molecular axes vertical to the mica substrate. In contrast to it, C(18)DAO molecules in monolayer folded on the substrate. For the equimolar mixture of arachidic acid and C(18)DAO, molecules were vertical in a part of the monolayer and folded in the other part. When the C(18)DAO monolayer was aged at 25 mN m(-1), molecules were rearranged with vertical molecular axes. Molecular ordering in arachidic acid and aged C(18)DAO monolayers was compared at a nanometer scale. Arachidic acid molecules ordered with a monoclinic unit lattice, whereas AFM could not detect a regular arrangement of C(18)DAO molecules.