Abstract A kinetic study of the fluoride ion catalyzed isomerization of a series of 2-arylF1- butenes shows the reactions to be pseudo first order in olefin at constant fluoride ion concentration. The resultant Hammett plot is non-linear witb a concave downward break near σ= 0. A two step mechanism involving formation of a carbanionic intermediate is proposed. A change in the rate limiting step causes the break in the Hammett plot. ...