The mechanism of intramolecular ketone-aromatic cyclodehydration of 1,3-diphenyl-1-propanones to give I-phenyl-1H-indenes in trifluoromethanesulfonic acid (TFSA) was investigated. The rate of the reaction of the monoprotonated diphenylpropanone increased proportionally to the acidity (H0) of the TFSA - trifluoroacetic acid matrix. This observation suggested that a species formed by a second protonation participates in the reaction. The O,O-diprotonated ketone is proposed as the true reactive intermediate. The substituent effect was also investigated.