The Langmuir layer behavior of arachidic acid/gamma-Fe2O3 nanoparticle complexes was studied at the air/water interface. The subphase was an aqueous colloidal solution (hydrosol) of gamma-Fe2O3 nanoparticles with an average diameter of 8.3 nm and with a standard deviation of +/-1.4 nm. Formation of the complex between arachidic acid and gamma-Fe2O3, nanoparticles was studied with surface pressure-area isotherms, surface potential-area isotherms and Brewster angle microscopy. Increasing surface pressure resulted in a transition from well-separated domains of the complex to well-compressed, nanoparticulate layers and, ultimately, to multiparticulate layers. The magnetic nanoparticles and layers of nanoparticles on solid substrates were studied with FTIR, Mossbauer spectroscopy and vibrating sample magnetometry (VSM). The gamma-Fe2O3 nanoparticles and Langmuir-Blodgett films with the nanoparticles showed superparamagnetic properties. The stability of the gamma-Fe2O3 nanoparticle hydrosol solution was studied by zeta potential measurements. Positively charged gamma-Fe2O3 nanoparticles in aqueous hydrosol solution at pH 3.5-5 showed excellent long-term colloidal stability.