Adsorption of chlorine on TiO2(110) surfaces modified by subsurface impurity atoms was studied by scanning tunneling microscopy. Adsorption was found to be suppressed in the vicinity of positively charged subsurface donor atoms. The reduced adsorption is explained by an increased electron affinity close to the impurity atoms that renders these sites energetically unfavorable for acceptor-like adsorbates. (C) 2002 Elsevier Science B.V. All rights reserved.