Cerium(III) and cerium(IV) both form formate complexes. However, their species in aqueous solution and the solid-state structures are surprisingly different. The species in aqueous solutions were investigated with Ce K-edge EXAFS spectroscopy. Ce(III) formate shows only mononuclear complexes, which is in agreement with the predicted mononuclear species of Ce(HCOO)(2+) and Ce(HCOO)(2)(+). In contrast, Ce(IV) formate forms in aqueous solution a stable hexanuclear complex of [Ce-6(mu(3)-O)(4)(mu(3)-OH)(4)(HCOO)(x)(NO3)(y)](12-x-y). The structural differences reflect the different influence of hydrolysis, which is weak for Ce(III) and strong for Ce(IV). Hydrolysis of Ce(IV) ions causes initial polymerization while complexation through HCOO- results in 12 chelate rings stabilizing the hexanuclear Ce(IV) complex. Crystals were grown from the above-mentioned solutions. Two crystal structures of Ce(IV) formate were determined. Both form a hexanuclear complex with a [Ce-6(mu(3)-O)(4)(mu(3)-OH)(4)](12+) core in aqueous HNO3/HCOOH solution. The pH titration with NaOH resulted in a structure with the composition (Ce-6(mu(3)-O)(4)(mu(3)-OH)(4)(HCOO)(10)(NO3)(2)(H2O)(3)]center dot(H2O)(9.5), while the pH adjustment with NH3 resulted in [Ce-6(mu(3)-O)(4)(mu(3)-OH)(4)(HCOO)(10)(NO3)(4)]center dot(NO3)(3)(NH4)(5)(H2O)(5). Furthermore, the crystal structure of Ce(III) formate, Ce(HCOO)(3), was determined. The coordination polyhedron is a tricapped trigonal prism which is formed exclusively by nine HCOO- ligands. The hexanudear Ce(IV) formate species from aqueous solution is widely preserved in the crystal structure, whereas the mononuclear solution species of Ce(III) formate undergoes a polymerization during the crystallization process.