Abstract The effect of ionic strength on the binding of aurothiosulphate to human serum albumin has been studied at 37° and neutral pH by equilibrium dialysis in unbuffered solutions. The effect of ionic strength is more pronounced on the lower association constants K 2– K 4 than on the high association constant K 1. Furthermore a reduction in the number of lower affinity binding sites is observed at low ionic strength. The main ionic strength dependence on the association constants agrees with the Debye-Hückel theory. The extrapolated values of K 1 and the sum of K 2 to K 4 at zero ionic strength are 7.6 × 10 5M −1 and 1.1 × 5M −1, respectively. It is shown that the observed changes in pH of the albumin solutions during dialysis contains valuable information of the aurothiosulphate-albumin interaction. A molecular binding mechanism is discussed.