The syntheses of cubic group 13 molecular phosphonate cages of the formulas [RPO3MR'](4) (R=Me, Et, t-Bu, Ph: MR'=BEt, BBu-s, GaMe, InMe; all combinations, 1-16), [t-BuPO3AlBu-i](4) (17), and [MePO3AlBu-t](4) (18) have been achieved by the facile reactions of alkyl- or arylphosphonic acids with the corresponding alkyl compounds under appropriately chosen reaction conditions. Compounds 1-18 have been characterized by analytical and spectroscopic techniques. In all cases, the central M4O12P4 inorganic cubic framework is shielded by alkyl/aryl groups. The phosphonates bearing tert-butyl groups on the phosphorus are highly soluble in common organic solvents such as n-hexane, Et2O, and THF. The molecular structure of representative examples [t-BuPO3BEt](4) (3) and [t-BuPO3BBu-s](4) (7) as determined by single-crystal X-ray diffraction studies reveal the presence of a central cubic M4O12P4 framework that is analogous to those found in alumino- and gallophosphate materials. Most of these cubic phosphonates are air and moisture stable. In the presence of excess phenol, it is possible to react one of the M-C bonds at the corners of the cube 3 and prepare its monophenoxy derivative [t-BuPO3-BEt](3)[t-BuPO3B(OPh)] (19) in very low yields. However, in spite the presence of reactive M-C bonds, these cubic phosphonates an unreactive toward alcohols, amines, carboxylic acids, H2O, and air under ambient conditions.