Utilizing a magnetron-sputtering (MagS) source in tandem with a multiple-ion laminar flow tube (MIFT) reactor and a customized triple quadrupole mass spectrometer (TQMS), we have prepared clean Pb-n(+) (n = 1-13) clusters and measured their reactivity with iodomethane under high carrier gas pressures. Strong size dependences are found for the reactivity of these cationic Pb-n(+) clusters with CH3I. For the Pb-n(+) with n <= 4, iodinated clusters PbnI+ were found to be the dominant products, in strong contrast to n > 4 where no such products were seen. Quantum chemical studies show that with an increasing number of Pb atoms, the Pb-Pb interatomic interactions become stronger compared with the Pb-I bonding in PbnI+ clusters. Furthermore, the reactions of Pb-1-4(+) with CH3I have fairly small transition state energy barriers, in contrast to those for Pb-n>4(+) clusters which have magnitudes that will prevent reactions under the ambient conditions.