The reaction of n-butane with "reactor equilibrated" (VO)(2)P2O7-based catalysts activated with different oxygen treatments has been investigated using high speed transient response techniques. It is shown that by maintaining the catalyst in a highly oxidized state the rate of maleic anhydride formation increases, and the n-butane selectivity to maleic anhydride improves. The activation energy for the reaction of n-butane was obtained by a kinetic analysis of the transient responses as a function of the temperature. The value of the activation energy depends on the oxidation state of the catalyst.