The effects of pressure were examined for the inclusion complexation of beta- and gamma-cyclodextrins (CDs) with 4-substituted phenols. The reaction volume for the CD inclusion complexation with 4-substituted phenols, which is the change in volume accompanied by CD inclusion, was estimated from the pressure dependence of the inclusion equilibria. ne reaction volumes estimated for the phenols/beta-CD system were positive values in the range of 2.5 and 14.0 cm(3) mol(-1), while those for the phenols/gamma-CD system were negative in the range of -11.8 and -0.8cm(3) mol(-1). The difference in the reaction volumes for the inclusion complexation of beta- and gamma-CDs is explained in terms of the difference in the number of water molecules repelled from the CD cavity. Based on the results, the inclusion behavior of CD complexation is discussed.