In this paper, we present the results of our investigations of the ionic reactions of methyl iodide, under multiple collision conditions as well as within the cluster environment. Utilizing a triple quadrupole mass spectrometer, the reactivity of various fragment ions (CD3+, I+, I2+, and CDxI+ (x = 0-3)) as they passed through a collision cell containing CH3I were studied for a range of low-energy collisions. It was observed that under conditions of low collision energy, only the I+ ions react with methyl iodide to generate I-2(+) ions. The reactions within methyl iodide clusters following electron-induced ionization were also studied. The various cluster ion sequences observed in the cluster mass spectrum of methyl iodide following electron-induced ionization can be rationalized on the basis of the ionic reactions of methyl iodide observed in the tandem mass spectrometric study.