The conditions of the formation of rare earth element 3,4-dihydroxybenzoates have been studied; their quantitative composition and solubilities in water at 293 K have been determined. The IR spectra and X-ray diffraction patterns of the hydrated complexes have been recorded and their thermal decomposition in air determined. During heating the hydrated complexes Ln(C7H5O4)3 . nH2O (n = 2, 3) lose water of crystallization in one step; then the anhydrous complexes decompose to the oxides with intermediate formation of very unstable Ln2(C7H4O4)3, with the exception of lanthanum which also forms La2O2CO3 as an intermediate.