The S-center radical (ANS(.)) of sodium 1-anilino-8-naphthalene sulfate (ANS) generated by photoinduced charge transfer in ANS/CdS and ANS/CdS/beta -cyclodextrin(beta -CD) systems has been studied by using spin trapping electron spin resonance techniques, UV-visible spectroscopic methods, and fluorescence spectroscopic methods. It was found that the S-centered radical (ANS(.)) was produced by the charge transfer reaction between the ground state ANS and the positive hole h(+)(CdS) from the valence band of CdS colloids, by the charge transfer from the excited singlet state (1)ANS* to the conduction band of CdS colloids, or by both in the ANS/CdS and ANS/CdS/beta -CD systems. The ESR signal intensity of the spin adduct (5,5 ' -dimethyl-1-pyrroline-N-oxide (DMPO)-ANS)(.), which is formed from ANS(.) trapped by DMPO, in the latter system is 15 times stronger than that in the former system. The apparent association constants between ANS and CdS colloids in the absence and presence of P-CD determined from fluorescence quenching experiments are 1097 and 1606 M-1, respectively. From ESR and fluorescence results, it is estimated that the efficiency of photoinduced charge transfer from ANS to CdS colloids in the ANS/CdS/beta -CD system is 12.5 times that in the ANS/CdS system.