Rate constants for oxidations of benzyl alcohol-d(0) and -d(7) by oxoiron(IV) tetramesitylporphyrin radical cation perchlorate in acetonitrile were measured in single turnover kinetic studies. The kinetic isotope effect (k(H)/k(D)) increased from 28 at 23 degrees C to 360 at -30 degrees C due to extensive hydrogen atom tunneling that was analyzed in terms of a parabolic energy barrier to tunneling. Similarly, large KIE values were found for oxidations of ethylbenzene-d(0) and -d(10) at room temperature. The large KIE values are a function of the porphyrin identity, and porphyrins containing electron-withdrawing groups display normal KIEs. KIEs found under catalytic turnover conditions are somewhat smaller than those obtained in single turnover reactions. The results should serve as benchmarks for computational studies of C-H oxidations by porphyrin and heme-iron-oxo systems.