A new method for controlling the hole density in single-walled carbon nanotube field-effect transistors (SWCNT-FETs) by solution-based chemical doping is presented. The use of organic molecules that adsorb onto SWCNTs from solution is investigated. The transfer characteristics of the SWCNT-FETs exhibit continuous and precise shifts in threshold voltages (see Figure) upon doping with F(4)TCNQ molecules, even in air.