Electron spin resonance (ESR) measurements on SWCNTs doped with alkali atoms (potassium) using a solid state reaction is reported. We find the emergence of the ESR signal of itinerant electrons upon doping with a signal intensity that is comparable to that expected from band structure calculations. The ESR line-width and microwave conductivity weakly depend on the temperature indicating taht doped SWCNTs are bad metals. It is argued to result from the lack of crystalline order in the tubes and the large impurity concentration. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim