The reduced viscosity of poly(ethylene glycol) (PEG) is increased in solutions of (2-hydroxypropyl)-alpha-cyclodextrin (HPACD) significantly more than in the solution of (2-hydroxypropyl)-beta-cyclodextrin (HPBCD) in agreement with the fact that a crystalline complex of PEG is formed in solutions of natural alpha-cyclodextrin but not in solutions of natural beta-cyclodextrin. In analogy with that, the viscometry indicates formation of a complex between HPBCD with poly(vinyl alcohol) because the reduced viscosity is markedly increased in solutions of HPBCD but only slightly in solutions of HPACD. On the other hand, the increase in the viscosity of poly(N-vinylpyrrolidone) (PVP) was identical in HPACD and HPBCD solutions; therefore, the viscometry did not provide support for the suggested side-chain complexation between PVP and cyclodextrins.