The adsorptive precipitation between eosin Y (EY) and AgCl colloids at pH 3.73 caused the sensitive color change of thesolution. The reaction mechanism between EY and AgCl was analyzed and this reaction was used for determination of Cl- in tracelevel by the light-absorption radio variation approach.2008 Hong Wen Gao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.