The indirect moving boundary method was used to determine the polyion constituent transport number in polyelectrolyte solutions containing mixtures of two monovalent counterions. The systems studied were mixtures of poly(styrenesulfonic acid) and its lithium or cesium salt with various equivalent fractions of the acid. All measurements were performed in water solutions at 25 degrees C in the concentration range from about 0.0025 to 0.1 mol/L. From the measured transport numbers and electrical conductivity of the mixtures, the fraction of free counterions was calculated for both ions present in the counterion mixture. The experimental results were compared with the theoretical predictions based on the cylindrical cell model. The dependence of the fraction of free counterions on the composition of the mixture is small but definite. It has been found, in agreement with the theory, that the fraction of free counterions for the smaller of the two ions increases and for the bulkier decreases with increasing its equivalent fraction in the mixture.