The kinetics of the isomerization of n-butane over a commercial sulfated zirconia catalyst have been studied. Experiments were performed in a fixed-bed reactor with both pure and diluted n-butane as a feed, at pressures between 4.6 and 8.1 MPa, and at weight hourly space velocities between 0.6 and 170 h(-1), which is equivalent to a range of modified residence times between 5000 and 71000 kg(.)s/m(3). Earlier studies have shown that the catalyst is stable under supercritical. conditions, however, in a narrow temperature regime only. Therefore, the reaction temperature was kept constant in this kinetic study at a value of 488 K. It is shown that the reaction is kinetically inhibited at high concentrations of n-butane in the feed, whereas mass transport may become a limiting factor in the highly diluted system. Three models will be presented, which are suitable for a robust and accurate description of the kinetics in broad pressure and concentration ranges.