Abstract Measurements of magnesium and calcium ion activities in solutions of the polyelectrolyte dextransulfate, with added sodium chloride or potassium chloride are presented. A two wavelength dye spectrophotometric method is used. Dextransulfate concentrations C p (expressed as moles sulfate ion/litre) vary between 0.001 and 0.007, total ionic strengths between 0.005 and 0.08 mole/XXX. Divalent metal ion concentrations are varied between 0 and 1.2 C p. The results for the metal ion activities are expressed in the form of parameters θ 2 = C 2 / C p ( C 2bp = bound divalent metal ion concentration) and K 2 = θ 2/( C 2- C 2b). For each divalent/univalent counterion pair the values obtained for θ 2 and K 2 as a function of C 2, C p, and ionic strength are compared to predictions of the “two variable theory” developed for these mixed counterion systems by Manning. This comparison shows that the observed decrease in θ 2 with increasing ionic strength at fixed C 2 and C p is generally well predicted by the two variable theory. The extent of divalent ion binding at a given C 2, C p, and ionic strength is largest for the Ca/Na counterion combination, and lowest for the Mg/K combination.