We describe here the synthesis and analysis of the reaction pathways leading to formation of the rare D5h decaphenylsilsesquioxane (SQ) [PhSiO1.5]10via F(-) catalyzed rearrangement of [PhSiO1.5]nn = 8, 12, and oligomers initially synthesized from PhSi(OEt)3. Isolated yields of ∼50% [PhSiO1.5]10 are obtained via rearrangement of all starting materials. The recovered starting materials can be re-equilibrated using catalytic F(-) to generate similar yields in second batches. These yields arise because [PhSiO1.5]10 exhibits higher solubility and better energy stabilization (10 kcal mol(-1) theory) in CH2Cl2 compared to [PhSiO1.5]8 or [PhSiO1.5]12. Reaction intermediates were identified using time dependent (19)F NMR and MALDI-ToF mass spectrometry eventually equilibrating to form the 8 : 10 : 12 cages in a 1 : 3 : 1.3 equilibrium in CH2Cl2. Experimental results coupled with modeling using the Gamess computational package provide multiple reasonable pathways for SQ rearrangements to [RSiO1.5]10, starting from [RSiO1.5]8. Heats of reaction for interconversion of the model intermediates [HSiO1.5]x determined computationally, were used to select the most reasonable reaction pathways. The findings support a mechanism involving activation and cleavage of a T8 cage corner by F(-) attachment, followed by the corners stepwise removal as [i.e. RSi(OH)3], followed thereafter by reinsertion forming [RSiO1.5]9-OH followed by, insertion of another corner to form [RSiO1.5]10-(OH)2 and finally condensation to give [RSiO1.5]10. The most enthalpically favorable path (-24 kcal mol(-1)) involves a hybrid mechanism.