Volatile hydrocarbons evolved by heating sulfated zirconia and iron promoted sulfated zirconias containing adsorbed 1-butene are detected and characterized by using temperature-programmed desorption coupled with mass spectrometry and with capillary gas chromatography-mass spectrometry. The presence of iron promoter has a dramatic effect on both the temperature at which volatile products evolve from catalysts and the composition of volatile hydrocarbon mixtures. These differences reflect differences in the reactivities of surface carbenium ions. Reactions on unpromoted and iron promoted catalysts are consistent with an apparent decrease in the Lewis acidity of surface carbenium ions when iron promoter is present.