Iron- and manganese-promoted sulfated zirconia (SFMZ) has been tested as an n-butane isomerization catalyst in the temperature range of 35 to 180 degrees C, The catalytic activity exhibits an induction period whose length is dependent on the reaction conditions, The presence of H-2 in the feed stream strongly suppresses n-butane conversion over unpromoted sulfated zirconia (SZ) and over Pt-containing SFMZ (PtSFMZ). However, hydrogen had no effect on n-butane isomerization over SFMZ. These findings were interpreted on the basis of a bimolecular mechanism where unsaturated intermediates (carbenium ions and/or butene) are formed during the break-in period, The role of promoters (Fe and Mn) is not only facilitating the formation of hydrogen-deficient intermediates and their accumulation on the catalyst surface, but also enhancing their stability. The negative effect of hydrogen over PtSFMZ is attributed to the occurrence of atomic hydrogen via the dissociative adsorption Of H-2 on Pt.