We report experimental and theoretical evidence of the different mechanisms that lead to doping of Si nanowires upon molecular adsorption of two paradigmatic Lewis bases. Pyridine genuinely dopes the nanowires by injecting charge carriers. Ethanol, on the other hand, simply modifies the dielectric screening conditions, allowing the reactivation of preexisting electrically passive impurities, and thus cannot control neither the nature (n- vs p-type) nor the concentration of the carriers. (C) 2013 AIP Publishing LLC.