Abstract: The FTIR spectra of fourteen rare earth tris (4,4,4-trifluoro-1-(2-thie-nyl)-1, 3-butanedione ) dihydrates in the 100-4000 cm^-1 range have beenstudied and the assignment of peaks in the far and middle infrared regionhas been discussed. Six RE--O absorption bands were observed, two ofwhich are in the mid--infrared region, namely 489--508 cm^-1 for the asym-metrical stretch and 456--475^-1cm for the symmetrical stretch. Four bandsare in the far infrared region, three of them are assigned to the metal-che-late oxygen bonds, while the fourth band results from the coordinationwith the water molecule. The frequencies of the bands near 456-489^-1cm change periodically with atomic number, showing the "tetrad effect", whichis characteristic of lanthanides. The absorption peak due to the carbonyl groupand that due to the C=C bond are lowered by about 60cm^-1 respectivelyin comparison with the free HTTA. This is an evidence of conjugation inthe metal chelate rings.