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We report in situ measurements of four-probe dc resistance (R) and thermopower (S) of Cs-and K-doped single-wall carbon nanotube (SWNT) mats as a function of a doping and temperature (T). With increasing dopant exposure, the mat resistance has been found to first decrease and then increase, exhibiting a minimum for optimal Cs doping. In contrast, for K doping, the mat resistance decreased monotonically and saturated. This unexpected result suggests that the diameter of the alkali-metal ion plays a role in the transport properties of the tube bundles. A doping-induced decrease in R by factors of∼ 120 and∼ 40 were observed for Cs-and K-doped SWNT mats, respectively. The low-temperature upturn of R (T) observed in all pristine SWNT samples was progressively suppressed with increased K doping. The optimally Cs-doped sample exhibited a positive dR/dT over the entire range of measurement (80 K< T …