The Taft's substituent constant of the pentafluorophenyl group (σ*(C6F5)) was reestimated to be 1.50 bycorrelation between IR spectral data (vCO) and σ* constantsfor a series of esters (involving the pentafluorobenzyl group)of 3-phenylpropanoic acid and butanoic acid. The possibilityof the disturbance of the correlation by the intramolecularπ−π interaction between C6F5 and C6H5 groups in pentafluorobenzyl 3-phenylpropanoate was excluded by ab initioand DFT calculations of the stable conformations and theircarbonyl frequencies. The reestimated σ*(C6F5) value wasused for calculation of the pKa value of pentafluorobenzylalcohol [14.5 (or 14.3)].