Reaction chemistry of an extremely sterically encumbered phosphinic chloride (Mes*)₂P(O)Cl (Mes* = 2,4,6-tri-t-butylphenyl, supermesityl) was investigated. This compound, as well as other compounds bearing two supermesityl groups placed geminally at the central phosphorus atom, shows extremely low reactivity at the phosphorus centre. Nevertheless, some synthetically significant transformations were possible. Reduction with hydridic reagents under forcing conditions yielded the phosphine oxide (Mes*)₂P(O)H and a secondary phosphine Mes*(2,4-tBu₂C₆H₃)PH. Deprotonation of (Mes*)₂P(O)H gave the corresponding phosphinite, which afforded very crowded tertiary phosphine oxides (Mes*)₂P(O)R (R = Me and Et) on reactions with electrophiles. While the reaction of the phosphine Mes*(2,4-tBu₂C₆H₃)PH with sulfur was surprisingly facile (although under forcing conditions), we have been unable to chlorinate or deprotonate this phosphine. All new compounds were fully characterised with multinuclear NMR, IR, Raman, MS, microanalyses and single crystal X-ray diffraction. Our computations (B3LYP and M06-2X level) show that strain energies of (synthetically accessible) geminally substituted compounds are extremely high (180 to 250 kJ mol⁻¹), the majority of the strain is stored as boat distortions to the phenyl rings in Mes* substituents.