The interactions between diquat (DQ) and p-sulfonatocalixarene (C4S) were studied in an aqueous solution as a function of the ionic strength. Evidence for the formation of a complex between DQ and C4S was obtained using fluorescence measurements, while the stoichiometry of the complex was confirmed as 1:1 for DQ/C4S using UV-vis spectroscopy. The ionic strength had no influence on the stoichiometry of the complex, but exerted a significant influence on the complexation constant, K-c, decreasing with an increase in the ionic strength. The thermodynamic complexation constant, K-c', was computed as 5.25 +/- 1.11 x 10(7) using the extended Debye-Huckel law. The rate constants for the heterogeneous electron transfer for the reduction of DQ at an electrode surface were evaluated as 0.150 +/- 0.010 cm s(-1) in the absence of C4S and 0.065 +/- 0.010 cm s(-1) when C4S was added to the solution in a 1:1 ratio. (C) 2012 Elsevier Ltd. All rights reserved.