New evidence for the influence of relativistic effects on coordination number, CN, and hydrolysis of Rf⁴⁺ and Db⁵⁺ aqua ions has been presented. It has been argued that very strong hydrolysis of the heaviest members of Groups 12, 13 is caused by the low CN of Hg²⁺ and Tl³⁺ in their aqua ions. Low CN is the result of relativistic effects which stabilize the 6s orbital, increasing thereby the respective promotion energy. That supports our previous view that strong hydrolysis of RF_aq⁴⁺ much greater than that of HF_aq⁴⁺ and Zr_aq⁴⁺, is also a result of lower CN. Since the promotion energy for sp³d⁴ hybridization (CN 8) seems to be close to that of sp³d² hybridization (CN 6), and since hydration energies of Zr⁴⁺ Hf⁴⁺ and Rf⁴⁺ do not differ very much for hexa- and octahydrates the two aqua ions i.e., M(H₂O)₆⁴⁺ and M(H₂O)₈⁴⁺ may coexist in aqueous acid solutions. lHowever, due to relativistic stabization of 7s and destabilization of 6d orbitals, which increases promotion energy, the Rf⁴⁺ aqua cation should show coordination number 6 rather than 8. A shift of equilibrium toward a lower hydrate increases the tendency to hydrolyse.