Abstract: The invention relates to a catalyst system for producing maleic anhydride by means of the catalytic oxidation of n-butane, comprising at least one reactor tube, which has two catalyst layers consisting of different catalyst particles, characterized in that the geometric surface area per catalyst particle is greater in the catalyst layer that is first in the gas flow direction than in the second catalyst layer. The invention further relates to a process for producing maleic anhydride by means of the catalytic oxidation of n-butane, wherein a mixture of oxygen and n-butane is fed through the catalyst system according to the invention and the at least one reactor tube is at elevated temperature.