The photoinduced interaction of fluorescein ester derivatives, fluorescein's butyl ester (FL4) and fluorescein's anthraquinone-methyl ester (FL-AQ), and colloidal US was examined by absorption, fluorescence spectroscopy, and photoinduced ESR spectroscopy. It is found that FL4 and FL-AQ molecules are adsorbed on the surfaces of US colloid by an electrostatic interaction forming the surface complex of the type CdS-FL4 or CdS-FL-AQ. The apparent association constant (K-app) and the degree of association (alpha) of CdS-FL4 obtained from absorption spectra are 2.25 x 10(4) M-1 and 0.78, respectively. The values of K-app and alpha of CdS-FL4 as determined from fluorescence spectra are 1.54 x 10(4) M-1 and 0.82, respectively, which matches well with that determined from the absorption spectra changes. And the values of Kapp and alpha of CdS-FL-AQ obtained from absorption spectra are 4.18 x 10(4) M-1 and 0.83, respectively. These data indicate that there is a strong interaction between the dye and the US particle surface. But there was no evidence for interfacial electron transfer from FL4 or FL-AQ to colloidal US by photoinduced ESR experiments. The fluorescence quenching is due to the formation of a nonfluorescent complex. The related phenomena are discussed in this paper. (C) 2003 Elsevier Inc. All rights reserved.