The oxidation by cytochrome P450cam (CYP101) of ethylbenzene and a series of substrates derived from it by addition of one, two, three, or four carbon atoms has been examined. For each of the 18 substrates, the shift in spin state due to substrate binding, the extent of coupled turnover to give organic products and uncoupled turnover to give H2O2 and H2O, and the identities of the organic products have been determined. The same studies have been carried out with the T185L and T185F mutants of P450cam in which the active site volume is decreased. The results show that no detectable correlation exists between the observed spin state change and any other parameter studied. For substrates of equal size, coupled and uncoupled turnover vary widely but both are maximized when the phenyl ring bears one large alkyl substituent or a methyl ortho to the largest alkyl substituent. The presence of substituents in addition to these decreases activity. In the absence of other changes, coupled turnover is correlated with the size of the largest substituent, but no such correlation exists for uncoupled turnover. Decreasing the size of the active site cavity by a T185L mutation generally increases coupled turnover without altering the dependence on the alkyl group size. A T185F mutation causes too great an active site perturbation for structure-activity studies. Substrate oxidation occurs preferentially at 2 degrees or 3 degrees C-H bonds of the largest substituent or on the benzylic methyl ortho to it. Aromatic hydroxylation only competes with the oxidation of nonbenzylic 1 degree C-H bonds. The extent of coupled turnover is a function of substrate shape, substrate size, and cavity size, but still elusive parameters control the extent of uncoupled turnover.