The reaction [Ru(NH3)(5)pz](2+) + CO(C2O4)(3)(3-) has been studied in aqueous solutions of p-sulfonatocalix[n]arene (n = 4, 6, and 8). The results are interpreted by using the pseudophase model. Results show that the rigidity and/or flexibility of the assembled rings have a great effect on the thermodynamics of inclusion of the guest into the host and, therefore, on the kinetics of the electron transfer processes that take place in these media. The obtained results are discussed from the viewpoint of two types of interactions: electrostatic and nonelectrostatic. From surface potential measurements, the guest-host interactions have been demonstrated to be mainly due to nonelectrostatic interactions, although the species are charged. So, the nonelectrostatic contribution to the equilibrium constant in all the calixarenes studied is I order of magnitude higher than the electrostatic one (K-nel = 144 and 884 mol(-1) dm(3) for p-sulfonatocalix[n]arene (n = 4 and 6, respectively) and K-el approximate to 10 mol(-1) dm(3)). Electrostatic interactions also affect the kinetic results.