Experimental partition coefficients of two ionic dyes in two-phase systems water+poly(ethylene glycol) 3000+dextran 500000 containing various salts are presented. Based on these data unambiguous information on the electrostatic potential differences induced by the salts is obtained. To achieve this goal the model proposed by Albertsson (1986) is applied to systems containing mixed salts to account for an ionic impurity present in the polymer samples. With both dyes the value of Delta phi(Na2SO4)-Delta phi(NaCl)=4.7 mV at salt concentrations up to 50 mmol kg(-1) is obtained. Also the partition coefficients of the salts are predicted in agreement with the experiment. These results indicate that up to a salt concentration of approximately 50 mmol kg(-1) the Albertsson model can be applied. Above this concentration limit, the basic two-phase system is altered and the model predictions are severely in error.