Density　functional　theory　calculations　were　performed　to　study　the　structures　and　relative　stability　of　the　gadolinium　complexes,　Gd(H　2O)n3+　(n8,9),　in　vacuo　and　in　aqueous　solution.　The　polarizable　continuum　model　with　various　radii　for　the　solute　cavity　was　used　to　study　the　relative　stability　in　aqueous　solution.　The　calculated　molecular　geometries　for　n8　and　9　obtained　in　vacuo　are　consistent　with　those　observed　in　experiments.　It　was　found　that　while　the　nona-aqua　complex　is　favored　in　the　gas　phase,　in　aqueous　solution　the　octa-aqua　conformation　is　preferred.　This　result,　independent　of　the　types　of　cavities　employed,　is　in　agreement　with　the　experimental　observation.　The　reliability　of　the　present　calculation　was　also　addressed　by　comparing　the　calculated　and　experimental　free　energy　of　hydration,　which　revealed　that　the　UA0,　UAHF,　and　UAKS　cavities　are　most　appropriate　when　only　the　first　solvation　shell　is　treated　explicitly.　©　2009　Chinese　Physical　Society.