The theoretical vibrational spectra of P4O6X (X = O, S, and Se) molecules are determined by employing the Hartree-Fock method in combination with a basis set of double-zeta quality plus polarization functions. A comparison of the three spectra is undertaken, and the shifts of the vibrational frequencies found in the spectra are analyzed. Two effects are considered to explain the changes in the band positions : the mass effect due to the replacement of oxygen by sulfur and selenium, respectively, and the bond strength effect. The mass effect of the substituent is as important as the different bond strengths of the P-V = X bonds. Previous assignments of the bands in the P4O7 spectrum are also corrected by the theoretical research, and a new correlation among the experimental vibrational spectra of the three compounds is put forth. Newly performed Raman polarization measurements confirm our theoretical findings.