We have shown that low-temperature C-13 NMR spectroscopy is an extremely powerful technique for characterizing the (iodomethyl)zinc-derived reagents involved in the cyclopropanation reactions. This technique has allowed us to spectroscopically characterize and unambigously differentiate the Furukawa reagent (EtZnCH(2)I), the Simmons-Smith reagent (IZnCH2I), and the Wittig reagent (Zn(CH2I)(2)). Unique spectra are obtained for each of these reagents when they are complexed to a chiral diether. We have also demonstrated that IZnCH2I is not converted into Zn(CH2I)(2) + ZnI2 in CD2Cl2 in the presence of a chiral complexing agent. Furukawa’s reagent "EtZnCH(2)I", however, is in equilibrium with Et(2)Zn and Zn(CH2I)(2), and it eventually decomposes into PrZnI and EtZnI at room temperature. The decomposition of Zn(CH2I)(2) into IZnCH2I and of IZnCH2I into ZnI2 was monitored by NMR. We have also demonstrated that the general trends observed for the various equilibria involving (iodomethyl)zinc-derived reagents follow those observed with ethylzinc-derived organometallic compounds.