In the present study , for the first time , a unique combination of in situ grazing incidence small-angle X-ray scattering and X-ray reflectivity , accompanied by the pressure–area isotherm analysis , Brewster angle microscopy , and ex situ scanning electron microscopy , was applied for investigation of two-dimensional superlattices of magnetic nanoparticles as they form on a water surface in a Langmuir trough. Iron oxide particles of different sizes stabilized with a single layer of oleic acid were used. It is demonstrated that monodisperse 10 nm particles on a water surface reproducibly form identical highly ordered monolayers in a wide range of experimental conditions , while monodisperse 20 nm particles always form compact three-dimensional clusters and never the monolayers. Monodisperse particles of an intermediate size , 15 nm in diameter , build a metastable monolayer , which shows a tendency for spontaneous transformation to bi- , tri- , and multilayer islands. The importance to use both grazing incidence small-angle X-ray scattering and X-ray reflectivity together with the complementary techniques , to avoid misinterpretation of separate experimental data sets , is underlined.