Measurements of the thermal membrane potential across cation exchange membranes were carried out by using aqueous solutions containing two 1-1 electrolytes, with an anion in common. The same solution was used on both sides of the membrane. In all cases a good linear relationship was observed between the thermal membrane potential delta psi and the temperature difference delta T (in the range delta T = +/- 10 degrees C). Assuming that the activity of one cation is equal to that of another cation in the solutions and the sum of transport numbers of cations is unity, the plot of delta psi/delta T vs logarithmic activity of one cation is linear with a slope of R/F. These experimental results are in agreement with a theory presented previously. From the analysis of thermal membrane potential in mixtures of electrolytes it is obtained that the cross coefficient of cation-cation interaction in membranes is negative and about 6 to 9% of the main coefficient.