Reactions of the zirconium amide guanidinates (R2N)(2)M[(PrNC)-Pr-i(NR2)(NPr)-Pr-i](2) (R = Me, M = Zr, 1; M = Hf, 2; R = Et, M = Zr, 3) with O-2 or H2O give products that are consistent with the oxo dimers {M(mu-O)[(PrNC)-Pr-i(NR2)-(NPr)-Pr-i](2)}(2) (R = Me, M = Zr, 4; M = Hf, 5; R = Et, M = Zr, 6) and polymers {M(mu-O)[(PrNC)-Pr-i(NR2)(NPr)-Pr-i](2)}(n) (R = Me, M = Zr, 7; M = Hf, 8; R = Et, M = Zr, 9). Mass spectrometric (MS) analyses of the reactions of water in air with 1 and 2 show formation of the Zr monomer Zr(=O)[(PrNC)-Pr-i(NMe2)-(NPr)-Pr-i](2), (10), oxo dimers 4 and 5, and dihydroxyl complexes M(OH)(2)[(PrNC)-Pr-i(NMe2)(NPr)-Pr-i](2) (M = Zr, 11; Hf, 12). Similar MS analyses of the reaction of diethylamide guanidinate 3 with water in air show the formation of Zr(=O) [(PrNC)-Pr-i(NEt2)(NPr)-Pr-i](2) (13), Zr(OH)(2)[(PrNC)-Pr-i(NEt2)(NPr)-Pr-i](2) (14), 6, and {(Et2N)Zr[(PrNC)-Pr-i(NEt2)(NPr)-Pr-i](2)}(+) (15). Kinetic studies of the reaction between 1 and a continuous flow of 1.0 atm of O-2 at 80-105 degrees C indicate that it follows pseudo-first-order kinetics with Delta H double dagger = 8.7(1.1)kcal/mol Delta S double dagger = -54(3) eu, Delta G(358) (K)double dagger = 28(2)kcal/mol, and a half-life of 213(1) min at 85 degrees C.