The Langmuir layer behavior of a polymer/magnetite nanoparticle complex at the air/water interface was studied beyond the liquid-expanded region with Brewster angle microscopy (BAM). The copolymer surfactant was poly(octadecene-co-maleic anhydride) (POMA) with an average molecular weight of 30 000. The subphase was an aqueous colloidal solution of Fe3O4 nanoparticles with an average diameter of 8.5 +/- 1.3 MI. In comparison to the case of a pure water subphase, the surface pressure versus specific area isotherm of the POMA was significantly expanded in the presence of the nanoparticles in the subphase. At 24 mN/m a phase transition was observed between the Liquid-expanded and the liquid-condensed states. In the absence of the magnetite nanoparticles the POMA monolayer exhibited no domains as observed with BAM. In the presence of nanoparticles, formation of domains, was observed at surface pressures of 24 mN/m or higher. The domain size increased with surface pressure, suggesting the growth of a two-dimensional solid complex in the liquid-compressed monolayer. Transmission electron microscopy (TEM) of the Langmuir layer deposited on a TEM grid showed the presence of magnetite particles.