The geminate reaction probabilities (for recombination and disproportionation) of benzoyl/sec-phenethyl radical pairs, generated by the photolysis of alpha-methyldeoxybenzoin, for both unlabeled (C-13 in natural abundance at the carbonyl position) and labeled ketones (C-13 in the carbonyl position) were measured in different sized alkyl sulfate micelles (sodium octyl sulfate (C-8) through sodium dodecyl sulfate (C-12)) in zero and high magnetic fields (B = 2400 G). Although the probability of geminate recombination (P(r)) diminishes for the unlabeled pair, from 0.549 to 0.436 and for the labeled pair from 0.585 to 0.504 at zero magnetic field with decreasing micelle size (C-12 to C-8), the efficiency of isotope separation (alpha) is found to increase at zero magnetic field from 1.144 to 1.236 with decreasing micelle size. Theoretical considerations of these experimental results show that the rate of geminate reaction of the unlabeled radical pairs in small micelles is sensitive to the electron spin exchange interaction; intersystem crossing is influenced by fast forced reencounters. These effects are not as important for the labeled radical pairs (which possess a strong C-13 hyperfine interaction). In addition, paramagnetic relaxation due to anisotropic hyperfine and electron-electron dipole interactions is shown to be important in the quantitative description of the micelle size dependence of the reaction ability of the labeled radical pairs.