The interactions between diquat (DQ) and p-sulfonatocalix[4]arene (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, Kc, decreasing with an increase in the ionic strength. The thermodynamic complexation constant, Kc′, was computed as 5.25±1.11×107 using the extended Debye–Hückel 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.