Neutron activation determination of 15–25 p.p.b. of tellurium in selenium can be done by irradiating 100-mg samples for 4 days at a neutron flux of 6 . ̄ 10 12 n. cm -2.sec -1. The daughter isotope of 131Te, 131I is separated by distillation and extracted into carbon tetrachloride. Finally the silver iodide precipitate is measured by β,γ -coincidence counting. The standard deviation is 3–7%.