The objectives of this work were to elucidate the mechanistic pathway of the title reaction, which constitutes the first example of a radical iodine atom transfer addition reaction of non-fluorine-containing phosphonates, and to determine whether 2-iodo-2-methylpropionitrile, 8, can serve as a competing iodine donor with the starting diethyl 1-iodoalkyl phosphonates, 1a,b. The title reaction was found to proceed with AIBN as the sole radical initiator, not requiring poisonous tin reagents as co-initiators, and gave diethyl 3-iodoalkylphosphonates 3a–e (the final products of the propagation step, isolated in 59–95% yield), tetramethylsuccinodinitrile, 9, diethyl methylphosphonate, 4 and tetraethyl ethylenebisphosphonate 5 (all termination products, 0–10% yields). The radical character of this reaction was demonstrated using TEMPO as a radical trap. 8 (the intermediate of the initiation step), synthesized independently from AIBN and iodine, caused complete inhibition of the reaction when added to the reaction mixture, indicating that it does not behave as an iodine donor in the transfer stage, but rather as an inhibitor.