which has been the subject of detailed kinetic studies for the solid-state salts K 4 [Ti(O 2 ) 4 ], K 3 [Nb(O 2 ) 4 ] and K 3 [Ta(O 2 ) 4 ]. Aqueous-phase decomposition is more complicated and pHdependent, but is known to involve the formation of similar oxyhydroxide complexes under some conditions. If the reaction depicted above is taken to be the principal degradation route for the anoinic systems considered here, the associated Gibbs free energy change can be used as a quantitative measure of complex stability. As presented in