The effect of polyelectrolyte counterion specificity on the interaction between dextran sulfate (DxS) with a charge density corresponding to 0.43, 0.7, 1..3, and 1.6 sulfate groups per monosaccharide unit, respectively, and the cationic amphiphile amitriptyline (3-(10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)-N,N-dimethyl-1-propanamine) was investigated by means of a dialysis equilibrium technique in water and aqueous/organic solvent mixtures. The binding isotherms for amitriptyline to DxS with 30 mM LiCl, NaCl, KCl, RbCl, and CsCl, respectively, followed the counterion sequence Li+, Na+ > K+ > Rb+ approximate to Cs+ i.e., at the same free amphiphile concentration, the degree of bound amphiphile decreased according to the order given. This sequence was retained in aqueous methanol as well as aqueous l-propanol mixtures. Furthermore, on addition of organic solvent, the critical aggregation concentration increased and the cooperativity of the amphiphile aggregation decreased.