Heterogeneous membranes of 2.0 cm diameter were prepared from a 60:40 mixture of thallium(I) molybdoarsenate and Araldite, These membranes gave near Nernstian response to thallium(I) ions over the concentration range 10−1to 10−3M but could be used for determination of thallium(I) down to 10−5M. Slope of the log concentration-potential plot is improved in 10 and 25 percent acetone-water mixture. The response was independent of pH over the range 4.0 to 6.0, The response time of the electrode is few seconds and potentials generated are reproducible. The selectivity of the electrode over a large number of cations was studied. The values of selectivity coefficients are of the order 10−2for monovalent and 10−3for bivalent cations. Anions did not interfere. Potentiometric-titration of thallium nitrate with potassium chromate was also done using the membrane as an end point indicator electrode.