The isomerization of undiluted supercritical n-butane over a commercial sulfated zirconia catalyst has been studied at temperatures between 443 and 533 K, pressures between 4.6 and 8.1 MPa, and weight hourly space velocities between 17 and 170 h(-1) (equivalent to modified residence times between 5000 and 42 000 kg s/m(3)). The reaction temperature is the decisive parameter with respect to catalyst stability. At temperatures above 500 K, the catalyst is desulfurized and deactivated. An operating area of steady-state processing could be identified below 500 K. The same catalyst suffers from rapid coke deactivation when the isomerization is carried out in the gas phase. Stable conversion levels realized with pure n-butane under supercritical conditions did not exceed 20%. However, because of the high throughputs, production capacities of isobutane are much higher under supercritical conditions than they are at atmospheric pressure using the same catalyst.