Brewster angle microscopy (BAM), x-ray specular reflectivity and grazing-incidence x-ray diffraction (GID) studies of C-60-propylamine adduct monolayers at the gas/water interface as a function of molecular area are reported. At large molecular areas (A>similar to 150 Angstrom(2)/molecule), BAM images reveal macroscopic heterogeneity in the film, consisting of the coexistence between regions covered with uniform solidlike monolayer and bare water surface. After compression to a limiting molecular area of 150 Angstrom(2)/molecule, the film is observed to be homogeneous, with the uniform monolayer covering the entire available surface. Both the x-ray reflectivity results and the GID patterns are consistent with the formation of a uniform monolayer at A similar to 150 Angstrom(2)/molecule, while the little dependence that the GID patterns have on the molecular area for A>similar to 150 Angstrom(2)/molecule is consistent with the heterogeneity in the film. Upon further compression to higher densities (A