Hydrolysis of the tris(tert-butoxy)siloxy complexes M[OSi((OBu)-Bu-t)(3)](4) (1,M = Zr; 2, M = Hf) was studied. Careful addition of 1 or 2 equiv of water to THF solutions of 1 and 2 produced the isolable aqua complexes M[OSi((OBu)-Bu-t)(3)](4)(H2O) (3, M = Zr; 4, M = Hf) and M[OSi((OBu)-Bu-t)(3)](4)(H2O)(2) (5, M = Zr; 6, M = Hf)., These complexes are stable for weeks in solution and are stable indefinitely in the solid state, In solution, the aqua ligands in 3-6 are labile. Crystals of 4 are monoclinic (C2/c) With a = 62.2352(4) Angstrom, b = 13.3362(2) Angstrom, c = 27.0940(4) Angstrom, beta = 103.535(1)degrees, V = 21836.0(4) Angstrom(3), and Z = 4. Addition of excess water to solutions of 5 and 6 at room temperature results in rapid hydrolysis to form a MOx(OH)(y)(OH2)(z) gel and HOSi((OBu)-Bu-t)(3). Below 10 degrees C in tetrahydrofuran-d(8), however, hydrolysis does not occur at an appreciable rate. At low temperatures, the rate of exchange between the aqua ligands of 5 and excess free water was found to depend on the water concentration This indicates that the aqua ligand exchange occurs by an associative mechanism involving a seven-coordinate intermediate. On the basis of this observation, we postulate that the hydrolysis of 5 also proceeds via the tris(aqua) intermediate Zr[OSi((OBu)-Bu-t)(3)](4)(H2O)(3). Compounds 3-6 are more thermally stable than 1 and 2, indicating that the elimination of isobutene from the latter complexes is facilitated by a free coordination site on the Lewis acidic metal center.