An ABA triblock copolymer, consisting of linear polyethylenimine (PEI) and poly(ethylene glycol) (PEG) (PEI-b-PEG-b-PEI), was examined for pH-dependent polypseudorotaxane formation with a-cyclodextrins (alpha-CDs). Although no complexation was observed for the mixture of alpha-CDs and PEI homopolymers, the polypseudorotaxane of PEI-b-PEG-b-PEI copolymer was prepared at pH below 8.0. H-1 NMR analysis showed that the stoichiometry for the polypseudorotaxane was about 2:1 ([EG + EI]: [alpha-CD]) at pH 11.0. Interestingly, the stoichiometry of [EG + EI] to [alpha-CD] below pH 8.0 was found to be about 4:1, which matched well with the assumption that the complex formation is ascribed only to the PEG block. X-ray diffraction and C-13 CP/MAS measurements verified that the control of pH affected the polypseudorotaxane formation, and the ionization state of the PEI block was the key factor to determine the threading and dethreading process of alpha-CDs onto polymer chains. Each block in the copolymer acted as a guest for alpha-CDs at pH 11.0, whereas alpha-CDs on the PEI block were dethreaded exclusively to result in block-selective polypseudorotaxane formation based on the PEG middle block at pH below 8.0, where the secondary amines in the PEI block was protonated (pK(a) 8.9). The threading/dethreading process of CDs along the PEI chains could be reversibly controlled by adjusting pH. Such unique pH-controllable polypseudorotaxane formation may be useful in designing many building blocks for stimuli-responsive polyrotaxanes.