a b s t r a c tThe 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 UVevis 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 0 , was computed as 5.25AE1.11Â10 7 using the extended DebyeeH€ uckel law. The rate constants for the heterogeneous electron transfer for the reduction of DQ at an electrode surface were evaluated as 0.150AE0.010 cm s À1 in the absence of C4S and 0.065AE0.010 cm s À1 when C4S was added to the solution in a 1:1 ratio.