The A(2)B(2) flavocytochrome p-cresol methylhydroxylase (PCMH) from Pseudomonas putida oxidizes 4-methylphenol (p-cresol) to 4-hydroxybenzyl alcohol in a process requiring scission of an a-C-H bond with concomitant reduction of covalently bound FAD in each A subunit. Values of k(cat)/K were determined from steady-state kinetic data for the reactions of PCMH with the following substrates: 4-methylphenol, 4-(H-2(1))methylphenol, 4-(H-2(2))methylphenol, and 4-(H-2(3))methylphenol. A procedure was devised to extract the intrinsic primary deuterium and intrinsic alpha-secondary deuterium kinetic isotope effects from these values of k(cat)/K. The primary effect, P, is 6.71 +/- 0.08, and the secondary effect, S, is 1.013 +/- 0.014. The magnitudes of these effects are discussed in terms of an early or late transition state, hydrogen tunneling, coupled motion between the leaving and remaining hydrogens of the methyl group, and a H- expulsion mechanism versus a substrate radical mechanism versus a covalent substrate-FAD intermediate mechanism. The reaction of 4-ethylphenol with PCMH produces 4-vinylphenol and (-)-S-1-(4-hydroxyphenyl)ethanol (similar to 100% enantomeric excess). The evidence indicates that these are formed from a common intermediate, presumably a p-quinone methide. From the partition ratios for the formation of the alcohol and 4-vinylphenol from 4-ethylphenol, 4-(1',1'-H-2(2))ethylphenol, and 4-(2',2',2'-H-2(3))ethylphenol, the primary isotope effect for conversion of the p-quinone (2',2',2' 2H3)methide to 4-(2',2'-H-2(2))vinylphenol was estimated to be about 2, and the a-secondary isotope effect for conversion of p-quinone (1'-H-2(1))methide to 1-(4-hydroxyphenyl)-(1'-H-2(1))ethanol was found to be inverse (=0.83), as expected for sp(2) to sp(3) hybridization change at the alpha-carbon. Values of k(cat)/K were determined for 4-ethylphenol, R,S-(+/-)-4-(1'-H-2(1))ethylphenol (abbreviated R,S-D), S-(-)-4-(1'-H-2(1))ethylphenol (S-D), R-(+)4-(1'-H-2(1))ethylphenol (R-D), and 4-(1',1'-H-2(2))ethylphenol (D2). The (D2)(k(cat)/K) value was found to be 5.1-6.1, the same as determined in an earlier study. Unexpectedly, the values for (R,S-D)(k(cat)/K), (S-D)(k(cat)/K), and (R-D)(k(cat)/K) were all about the same (similar to 1.7), indicating that then is nearly an equal probability for pro-R or pro-S C-H bond scission. An apparent flux ratio for the pro-S path/pro-R path was estimated to be 0.78 +/- 0.02. The same procedure devised to determine values for P and S for 4-methylphenol was used to determine these values for the 4-ethylphenol reaction (commitment to catalysis = 0); P = 5.98 +/- 0.12 and S = 0.967 +/- 0.021. These values are essentially the same as those determined for 4-methylphenol. Thus, the chemical mechanisms for both substrates are assumed to be similar.