Abstract: Metathetical reaction of rare earth chloride and sodium 10-undecenoic salt gave a number of precipitates with chemical composition of REA₃·nH₂O(A represents the deprotonated form of the acid). 15 such a compound have been prepared and reported in this paper. They are all insoluble in common organic solvents except chloroform. The conductivities of the salts in chloroform show that they are all non-electrolytes. IR spectra suggest that they are mainly ionic compounds. Antibacterial activities of cerium 10-undecenoic salt also have been tested.