Ion-cyclotron resonance mass-spectrometric experiments are used to examine the reactions of small cationic vanadium clusters V-n(+) and vanadium-oxide clusters VnOm+ (n less than or equal to 5, m less than or equal to 3) with molecular oxygen. Most of these reactions are very fast (k greater than or equal to 3 x 10(-10) cm(3) molecule(-1) s(-1)) and strongly exothermic. Oxidative degradation of the clusters (decreasing n) is generally more favored than oxygen-transfer resulting in VnOm+ cations (constant n). Consequently, the mononuclear oxide ions VO+ and VO2+ evolve as the major final products at the end of a reaction cascade. For V-2(+) and V-3(+), the complete network of elementary steps is explored by MS/MS experiments. Furthermore, some thermochemical brackets are derived, e.g., for the enthalpies of formation of the ions V2Om+ (m = 1-3): 920 kJ/mol less than or equal to Delta(f)H (V2O+) less than or equal to 1130 kJ/mol; 670 kJ/mol less than or equal to Delta(f)H (V2O2+) less than or equal to 880 kJ/mol, and 420 kJ/mol less than or equal to Delta(f)H (V2O3+) less than or equal to 630 kJ/mol.