gamma-alumina catalysts modified with different weight loadings of fluorine have been used for skeletal isomerization of l-butene in order to investigate the effects of the fluorine loading level on the conversion of I-butene and the selectivity to isobutene formation. Increasing the actual loading of fluorine up to 0.012 wt% led to an increase in conversion of I-butene over fluorine-modified gamma-alumina catalysts, while the high selectivity to isobutene remains almost unchanged. On the other hand, a clear trend of increasing l-butene conversion with a decreasing selectivity to isobutene is observed for the gamma-alumina catalysts with higher loadings of fluorine. An analysis of the results from the thermal analysis, NH3 temperature-programmed desorption, infrared and the I-butene sorption measurments clearly indicates that the number of strong acid sites in the modified gamma-alumina catalysts is greatly enhanced at fluorine loadings higher than 0.012 wt%, leading to the acceleration of 1-butene oligomerization followed by cracking to light hydrocarbons. Therefore, the I-butene isomerization selectivity from fluorine-modified gamma-alumina catalysts can be understood in terms of a competition between the monomolecular and bimolecular reaction pathways, which highly depend on the concentration of strong acid sites.