Thirty-three variants of the flavoprotein component of p-cresol methylhydroxylase that contain noncovalently or covalently bound flavin adenine dinucleotide (FAD) analogues were studied. A very good correlation was found between the efficiency of p-cresol oxidation by these proteins and E-CT, the energy for the maximum wavelength for the charge-transfer band of the complex between the bound flavin and 4-bromophenol, a substrate mimic. The correlation covers a range of k(cat) values that spans over 5 orders of magnitude and values of E-CT that span 900 mV, and the analysis of the data provided a value of the transfer coefficient, alpha, of 0.31. This study demonstrates clearly that the redox properties of both the bound substrate and the flavin cofactor must be taken into account to explain the relative catalytic efficiencies of the variant flavoproteins.