The first detection of bidirectional inclusion by water-soluble calixarene is reported. The free-radical probe alpha-(2,4,6-trimethoxyphenyl)benzyl tert-butyl nitroxide is shown to produce two isomeric inclusion complexes with p-sulfonatocalix[8]arene in aqueous media, being detected with ESR spectrometry. ESR spectra were assignable to tert-butyl-in and phenyl-in complex and the association constants at room temperature were 131 and 45 mol(-1) dm(3) for tert-butyl-in and phenyl-in, respectively.