The radical pair that results from photolysis of adenosylcob(III)alamin (AdoCb1(III)) undergoes primary geminate recombination with a first-order rate constant of 1 x 10(9) s(-1). In contrast, methylcob(III)alamin (CH(3)Cb1(III)) and aristeromicylcob(III)alamin (AriCb1(III), the carbocyclic analogue of AdoCb1(III) in which the ribofuranose ring oxygen has been replaced with a methylene group) does not undergo primary geminate recombination. The ribofuranose group enables a high rate of geminate recombination in the [Ado . Cb1(III)] radical pair. This may be due to a stereoelectronic (beta-anomeric) effect that maintains a pyramidal geometry at the 5’-carbon of the 5’-deoxyadenosyl radical, or it may be due to hindered rotation about the C-4’-C-5’ bond such that beta-elimination to the olefin is prevented. Recombination in the geminate singlet radical pair is in competition with diffusive escape to form a solvent-separated radical pair. Hyperfine coupling from Co-II promotes intersystem crossing to the triplet radical pair (Chagovetz, A. M.; Grissom, C. B. J. Am. Chem. Sec. 1993, 115, 12152). Recombination of the [CH3 . Cb1(II)] radical pair is not prevented by a lack of intersystem crossing, as neither unlabeled or C-13-labeled CH(3)Cb1(III) undergoes geminate recombination. There is only a small difference in the rate of diffusive recombination in the solvent cage for AdoCb1(III), AriCb1(III), and CH(3)Cb1(III) following photolysis : 2.01 x 10(4) s(-1), 2.20 x 10(4) s(-1), and 1.16 x 10(4) s(-1). The rate of diffusive recombination is limited by productive collisions and not by radical geometry or intersystem crossing. The CF3 . radical that results from photolysis of (trifluoromethyl)cob(III)alamin (CF(3)Cb1(III)) maintains its pyramidal geometry and undergoes faster diffusive recombination in the solvent cage at 51 x 10(4) s(-1). The C-Co bond dissociation enthalpy in AriCb1(III) is 37 +/- 1.4 kcal/mol. The profound difference in geminate recombination rates for AdoCb1(III) and CH(3)Cb1(III) is consistent with their different biological roles as enzymatic cofactors : AdoCb1(III) is an initiator of radical chain chemistry in the active site, whereas CH(3)Cb1(III) is a methyl group donor in an S(N)2-type process.