Four novel Polyhedral Oligomeric Silsesquioxanes (POSSs) having seven C6H5- groups linked to seven silicon atoms of cage, whilst a substituted -p (C6H4-X) group (X = -CH3, -OCH3, -F, -Cl) is linked to the eighth silicon atom, were synthesized and characterized by elemental analysis and H-1 NMR. POSS5 were degraded in both inert and oxidative atmosphere, and the temperatures at 5% mass loss (T-5%) were determined as a measure of the resistance to thermal degradation. Differential Scanning Calorimetry (DSC) and solubility in CHCl3 and THE experiments were also performed and the results obtained were compared with those found for commercial octaphenyl POSS, which was also here studied. Mono substituted POSS5 exhibited slightly lower T-5% values and higher solubility than octaphenyl POSS. This behaviour was attributed to symmetry changes determined by the substitution of a hydrogen atom of a POSS phenyl group with various dimensions substituents.