The effects of fatty-acid molecules (CxH2xO2, x = 10, 12, 14, 16, 18, 20) on Fe-Pt nanoparticle arrangement by Langmuir-Blodgett method are studied. The surface pressure-area (pi-A) isotherm for the Fe-Pt nanoparticle-layer with fatty-acid molecules reveal that the molecules form a buffer layer between the Fe-Pt nanoparticle-monolayer and water to reduce the surface energy in the case that x is equal or superior to 14, resulting in the regular arrangement of the nanoparticles. (C) 2005 Elsevier Ltd. All rights reserved.