The chemical treatment of single wall carbon nanotube (SWNT) samples leads to a modification of their vibrational and electronic properties. We here report the alteration of the Fermi level of SWNTs chemically modified with Ag and CrO3 as observed in the resonance Raman spectra of both metallic and semiconducting SWNTs. Our results indicate that while Ag behaves as an electron donor, CrO3 behaves as an electron acceptor for SWNTs. (C) 2003 Elsevier B.V. All rights reserved.