X-ray reflectivity measurements and surface-pressure isotherms of spread films of the fullerene-propylamine adduct (C-60-[NH2(CH2)(2)CH3](12)) indicate that despite the lack of definite amphiphilic character, this material forms a homogeneous monomolecular layer on water surfaces. X-ray reflectivity data, taken at various lateral pressures, were fitted to a model structure defined in terms of the dimensions of an average cell and its chemical compositions. At T = 18 degrees C, the lateral pressure versus area per molecule is indicative of a structural phase transition presumably from one closely-packed phase to another, as the total thickness of the film is consistent with a single layer of 15 Angstrom thickness. At pressures above 30 mN/m the film consists of an inhomogeneous bilayer, consistent with a layer thickness of 26 Angstrom.