The infrared and polarized Raman spectra of the trigonal Li 6 P 6 O 18 ·3H 2 O crystal are reported. The results are analysed using several group theory approaches, in terms of internal and external modes of the highly symmetric P 6 O 18 cyclophosphoric ring and water molecules. Equilibrium geometries and vibrational spectra of P 6 O 18 units, free and in interaction with watermolecules (P 6 O 18 ·3H 2 O) have been determined by ab initio calculations using the basis set 6-31+G(d) of Hartree Fock method. Experimental frequencies and polarisation conditions are remarkably consistent with ab initio calculations. A detailed description of the normal modes of vibration of these systems is presented.