A new polyrotaxane was formed by the polycondensation of p-tert-butylphenol (BP) with paraformaldehyde in the presence of poly(ethylene glycol)s, (PEG)s, of various molecular weights. The polyrotaxane of BP oligomers with PEG was obtained when the PEG of average molecular weights between 1000 and 10 000 were used in the polycondensation. The polyrotaxane yield and the composition were dependent on the molecular weight of PEG. The molecular structure was determined by FT-IR and H-1 NMR spectroscopies and the thermal properties by DSC measurements. H-1 NMR spectra of the complexes showed a pair of doublets in the methylene region, which correspond to the methylene protons between phenolic rings in p-tert-butylcalixarenes. The FT-IR spectra and DSC curves were different from those of the blend of linear p-tert-butylphenolic resins with PEG and pure p-tert-butylcalix[8]arene, respectively. Calixarenes slipped off the backbone PEG by heating the polyrotaxane in ethylene glycol at 180 degreesC. The structure of the dethreaded calixarenes was determined by H-1 NMR, and they were found to be p-tert-butylcalix [8]arenes and a few p-tert-butylcalix[4] arenes. From the characteristic results it was found that the polyrotaxane obtained by the in situ polycondensation of BP was composed of p-tert-butylcalix[8]arenes penetrated by PEG; that is, this was a calix[8]arene-based polyrotaxane.