The La(III), Y(III), Ce(III) and Sm(III) rare earth carbonates, with general formula, M2(CO3)3·xH2O, have been synthesized by a new synthetic method involving the reaction of aqueous solutions of the La(III), Y(III), Ce(III) and Sm(III) sulfate salts with urea as precursor at ~90 oC. A study of the IR absorption spectra (4000-400 cm-1) of the formed products clearly indicates the absence of bands due to urea and shows the characteristic bands of carbonate ion. A general mechanism describing the formation of La(III), Y(III), Ce(III) and Sm(III) carbonate hydrates, M2(CO3)3·xH2O, is suggested.