Inhibition of one-electron oxidation of 1-pyrenesulfonate (PySA(-)) included in cyclodextrins by sulfate radical anion (SO4.-)occurred to give PySA- radical cation (Py(+)SA(-)) with a peak at 455 nm during the pulse radiolysis of a mixture of PySA(-), potassium peroxodisulfate, and t-butanol. PySA(-) is partly included in beta-cyclodextrins (betaCD) to form a PySA(-)/betaCD complex, while it exists only as a 2:2-inclusion complex. (PySA(-))(2)/(gammaCD)(2) in gammaCD. The oxidation of PySA(-) by SO4.- was completely inhibited in betaCD and gammaCD. This is explained by the shielding effect of CDs on the bimolecular collisional process of PySA(-) and (PySA(-))(2) in CDs with SO4.-. (C) 2004 Elsevier B.V. All rights reserved.