In the present work the bonding situation within the P4On (n = 6-10) series is studied using a combination of experimental and theoretical vibrational analysis. A correlation between the spectra of the compounds is undertaken, and the shifts of the vibrational frequencies within the series are analyzed. Our study shows that the frequencies of the most modes involving cage motions increase, when more oxygen atoms are added to the phosphorus centers. This reflects the reinforcement of the cage bonds along the P4On (n = 6-10) series. For a few bands the frequencies decrease within the series, and this fact results from the reduced masses, which become larger with the increasing number of the substituents.