The effects of pressure were examined for the inclusion complexation of water-soluble p-sulfonated calix[n]arenes (Calix-Sn: n = 4, Calix-S4; n = 6, Calix-S6; n = 8, Calix-S8) with methylene blue (MB). A differential pressure effect was found in the inclusion equilibria of Calix-Sn. As the external pressure increases, the inclusion equilibrium of MB with Calix-S4 and Calix-S6 shifts to the dissociation side. However, with Calix-S8, the inclusion equilibrium shifts to the association side with increased pressure. From the pressure dependence of the inclusion equilibria of Calix-Sn, the reaction volumes, which are changes in volume accompanied by inclusion, were estimated as 32.1 cm(3) mol(-1) for Calix-S4, 30.1 cm(3) mol(-1) for Calix-S6 and -13.8 cm(3) mol(-1) for Calix-S8. In addition, the structures of the inclusion complexes of Calix-Sn with MB have been established by H-1 NMR measurements. Based on the results, the behavior of inclusion complexation with Calix-Sn is discussed. Copyright (c) 2004 John Wiley T Sons, Ltd.