Monolayers of newly synthesized complexes of lanthanoids, such as samarium(III), europium(III), and terbium(III), containing two long alkyl chains and their mixtures with arachidic acid or octadecane were investigated. These complexes can form stable condensed monolayers having molecular areas of 104-111 Angstrom(2) at the air-water interface. Octadecane molecules in the mixed monolayers assist close packing not only of long alkyl chains but also for large head groups. The mixed monolayers were deposited on quartz plates by the Langmuir-Blodgett method and a horizontal lifting technique. It has been found that in the monolayer assemblies of europium(III) complexes, the emission probability from the higher excited singlet state D-5(1) and the Symmetric forbidden transition D-5(0) --> F-7(0) is enhanced in comparison with that in solution, and further the emission of D-5(0) --> F-7(0) splits into two peaks. These results are ascribed to the fact that the thermal deactivation of the higher excited state is decreased and the symmetries of these complexes are slightly distorted in the monolayer assemblies. Characteristic emission behavior for the samarium(III) and terbium(III) complexes was observed depending on the different molecular assembly systems, though these effects were not as significant as in the case of the europium(III) complex.