The synthesis, isolation, and characterization of (pentafluorophenylthiolato)cobalamin, C(6)F(5)SCbl, are reported, only the second isolable RSCbl known. The synthesis of C(6)F(5)SCbl in 81% yield is accomplished by the dropwise addition of 0.93 equiv of C6F5SH in MeOH under argon to 1 equiv of HOCbl . HOAc (50.7 mg) in MeOH, all at -15 degrees C and under darkened, red-light-only conditions. The desired C6F5SCbl product was precipitated by the addition of cold, degassed acetone, isolated, and then characterized by FABMS and UV-visible, H-1 NMR, and F-19 NMR spectroscopies. Also reported is a H-1 NMR method for the detection of impurities in cobalamins. With the requisite, isolable RSCbl precursor to RS. and (.)Cbl radicals now in hand, the needed chemical precedent studies of the proposed H-. abstraction step from the substrate in ribonucleoside triphosphate reductase by a RS. analogue of the protein-S-. site an now possible.