A kinetic study has been made of the cycliacylation in liquid HF of protonated 3-phenylpropanoic and 4-phenylbutanoic acids and a number of their alkyl derivatives to the corresponding 1-indanones and 1-tetralones*. The acylating species in these reactions has been shown to be the free acyl cation. Formation of this cation becomes rate-determining at high rates of the cyclization step proper and then masks any further activation of the benzene ring. The cyclization to 1-indanones, but not that to 1-tetralones, exhibits an unusually large ortho partial rate factor (for methyl groups fo is as large as fp). PMR spectroscopic data indicate that replacement of the benzylic hydrogen atoms in 3-phenylpropanoic acids by methyl groups has a great influence on the conformation around the C(2)-C(3) bond. This results in a proximity effect that accelerates the cyclization reaction by a factor of about 102.