alpha-Deuterium (alpha-D) kinetic isotope effects (KIEs) have been determined for the reaction of methyl iodide with a series of reducing agents. Reagents which transfer hydride ion in an S(N)2 reaction show small inverse or small normal KIEs. Reagents which transfer an electron to methyl iodide to produce methyl radical show large normal KIEs up to 20% per alpha-D. Large KIEs were found for the reaction of methyl iodide with sodium, for Pd-catalyzed reaction of methyl iodide with hydrogen, for electron transfer (ET) at a platinum cathode, for ET from benzophenone ketyl or from sodium naphthalenide, for iron-catalyzed ET from a Grignard reagent to methyl iodide, and for reduction of methyl iodide with tributyltin hydride or with gaseous hydrogen iodide. Very small KIEs were found for electron transfer to methyl iodide from magnesium in ether or from sodium in ammonia. The reason may be that these reactions are transport or diffusion controlled.