By combining Raman spectroscopy with transistor transfer characteristics, acid treatment of single-walled carbon nanotubes (SWCNTs) in a mixture of concentrated HNO3/H2SO4 has been characterized. The acid treatment results in a sharp decrease in the Raman resonant signals of the metallic SWCNTs but no observable change in those of the semiconducting SWCNTs. However, the acid treatment causes disappearing gate modulation of the thin-film transistors made of the SWCNTs, contrary to what would be expected referring to the Raman results. These experimental results suggest that the energy band of the semiconducting SWCNTs is significantly affected by absorbates induced by the acid treatment. (C) 2009 Elsevier B.V. All rights reserved.