A potentiometric method utilizin g gla ss me mbrane refere nce e lec trod es in a co nce ntration ce ll for the continuous measure me nt in s itu of molte n sa lt co mpos iti on is presented . Re leva nt da ta for th e AgCI-NaCI, AgBr-NaBr, and Ag,SO,-Na2S04 sys tems are give n. Us ing ce ll s of the type M\ IM\A(all), M2 A(a,;)lgla ssI M\A(a,). M,A(aJ IM\where the a' s a re ac tiviti es, the unkn ow n mole fraction X, ca n be ca lc ulated from the eq uation2.303(RT/F)I, -log x;;-wh e re t' is a conce ntra ti on-ind epen de nt cons tant of th e sys te m a nd XII is the conce ntration of M \A in th e referen ce e lec trode. The me th od is capa bl e of measuring X, in molte n salt with an accuracy of better than 5 percent over a co nce ntration range of 10 4App li cation of th e method to ca tions othe r than s ilve r is di sc ussed.