Treatment of V(N[Bu-t]Ar)(3) (1) (Ar = 3,5-Me2C6H3) with O-2 was shown by stopped-flow kinetic studies to result in the rapid formation of (eta(1)-O-2)V(N[Bu-t]Ar)(3) (2) (Delta H-double dagger = 3.3 +/- 0.2 kcal/mol and Delta S-double dagger = -22 +/- 1 cal mol(-1) K-1), which subsequently isomerizes to (eta(2)-O-2)V(N[Bu-t]Ar)(3) (3) (Delta H-double dagger = 10.3 +/- 0.9 kcal/mol and Delta S-double dagger = -6 +/- 4 cal mol(-1) K-1). The enthalpy of binding of O-2 to form 3 is -75.0 +/- 2.0 kcal/mol, as measured by solution calorimetry. The reaction of 3 and 1 to form 2 equiv of O V(N[Bu-t]Ar)(3) (4) occurs by initial isomerization of 3 to 2. The results of computational studies of this rearrangement (Delta H = 4.2 kcal/mol; Delta H-double dagger = 16 kcal/mol) are in accord with experimental data (Delta H = 4 +/- 3 kcal/mol; Delta H-double dagger = 14 +/- 3 kcal/mol). With the aim of suppressing the formation of 4, the reaction of O-2 with 1 in the presence of (BuCN)-Bu-t was studied. At -45 degrees C, the principal products of this reaction are 3 and (BuC)-Bu-t(=O)N V(N[Bu-t]Ar)(3) (5), in which the bound nitrile has been oxidized. Crystal structures of 3 and 5 are reported.