The electrical conductance of individual single-wall carbon nanotube (SWCNT) transistors was investigated before, during, and after soaking in concentrated sulfuric acid in air environment. The resulting doping effect has induced a shift of the Fermi level which was monitored by the electrical behavior and sought in Raman spectroscopy signature. For either metallic or semiconducting SWCNTs, the maximum conductance value was lowered during the contact with the acid; remained low for a short time after removing the acid; and recovered after long air exposure. This demonstrated the significant contribution of the physisorbed species in the measurements, specifically at the metal pad/SWCNT contact lines. The study also demonstrated that washing with sulfuric acid is an easy method for removing adsorbed species and reaching the intrinsic conductance of SWCNT-based devices.