Abstract: Laser Raman spectra of the rare earth acetylacetonate anhydrous, mono and trihydrated complexes indicated that hydrated molecules have effects on the M-O stretching vibration bands of these complexes. It was found that anhydrous complexes have only one band near 400cm^-1, mono-hydrated complexes have two bands of the different intensities near 400 and 415cm^-1, and that the M-O stretching vibration frequencies of them change periodically with atomic number, showing the "tetrad effect". However, the band near 400cm^-1 of trihydrated complexes undergoes deformation and different changes with the light and heavy rare earth ion, therefore, the "tetrad effect" characteristic of lanthanide was disturbed and destroied. When monohydrated complexes loses coordination water molecule, the band near 415cm^-1 disappears, and if dehydrated complex absorbs water molecule in air, this band again appears. Assignment of the band near 415cm^-1 in the last paper was modified on the basis of the above-mentioned invertible experimental fact, it is due to ( M-O ) H₂.