Europium(III) complexes with P-diketones containing various substituents were newly synthesized and their monolayer behaviors on the water surface were studied in situ by a Brewster angle microscopy (BAM) together with surface pressure-area isotherms. Some BAM images look like a thin soap him on the flat surface, consisting of gas and liquid phases. The monolayer assemblies of these complexes could be deposited by both LB and horizontal lifting techniques. The emission probability from the excited singlet state D-5(1) increased in the film as compared to the lowest excited state D-5(0), and the symmetrically forbidden transition D-5(0)-->F-7(0) was enhanced in comparison with those in the solutions and the crystallines. This effect on the fluorescence was observed significantly for the complex with an asymmetrically substituted ligand rather than that with a symmetrically substituted one. These results can be ascribed to the fact that the thermal deactivation of the higher excited state is decreased and also the symmetries of these complexes are slightly distored in the monolayer assemblies.