The hydrolysis constants of Gd(3+) and the solubility products of Gd(OH)(3) and Gd(OD)(3) in nitrate solutions at 25 and 70 degrees in H(2)O and D(2)O have been determined because of their importance in nuclear technology. The constants are defined (charges omitted for clarity) as *K(11) = a(GdOH)a(H)/a(Gd), *K(21) = a(Gd(OH)(2))a(2)(H)/a(Gd), *K(SO) = a(Gd)/a(3)(H). The values for the H(2)O system were p*K(11) = 7.87 +/- 0.02, p*K(21) = 15.I6 +/- 0.09, p*K(SO) = -19.32 +/- 0.03 at 25 degrees and p*K(11) = 7.55 +/- 0.03, p*K(21) = 13.04 +/- 0.03, p*K(SO) = -16.16 +/- 0.04 at 70 degrees . For the D(2)O system they were p*K(D)(11) = 8.17 +/- 0.01, p*K(D)(21) = 16.00 +/- 0.09, p*K(D)(SO) = -21.18 +/- 0.04 at 25 degrees and p*K(D)(11) = 7.84 +/- 0.02, p*K(D)(21) = 13.95 +/- 0.02, p*K(D)(SO) = -17.34 +/- 0.04 at 70 degrees . The mean enthalpy changes of the reactions were also calculated.