Gluconate is a multidentate ligand and its complexation with actinides has received increasing attention because of its existence in high level nuclear wastes as well as in nuclear waste repositories. In this work, the complexation of gluconate with Th(IV) was studied in deuterated water (D2O) by pD titrations and nuclear magnetic resonance (NMR) spectroscopy. In the pC(D) range 2.0-4.6, gluconate (Ga-4(-)) forms two 1:1 complexes with Th(IV), Th(GD(3))(2+) and Th(GD(2))(+). Their stability constants were determined to be log beta((D))(101(-1)) = 1.04 +/- 0.12 for Th4+ + GD(4)(-) = Th(GD(3))(3+) + D+ and log beta((D))(101(-2)) = -(1.31 +/- 0.09) for Th4+ + GD(4)(-) Th(GD(2))(+) + 2D(+) at I = 1.0 mol.L-1 NaClO4 and t = 22 degrees C. The coordination modes of these two complexes were also analyzed. In both complexes, the tridentate chelation forms through the binding of Th(IV) to one oxygen from the carboxylate group and two oxygens from alpha- and gamma-hydroxyl groups. The difference is that in Th(GD(2))(+), both alpha- and gamma-hydroxyl groups deprotonate, and in Th(GD(3))(2+), only the alpha-hydroxyl group deprotonates.