This paper describes a Neutral Particle Analyzer/Isotope Separator(ISEP) developed for measurement of relative hydrogen isotope composition of JET plasmas. The ISEP deployed on JET can be regarded as a prototype of an instrument proposed for measurement of spatial profile of ratio of density of deuterium and tritium ions in the plasma, nD(r)/nT(r), in the International Thermonuclear Experimental Reactor (ITER). ISEP makes simultaneous measurements of energy distribution of efflux of hydrogen isotope atoms (H, D and T) from the plasma. From such measurements it is possible to deduce the radial profile of relative hydrogen isotope ion composition of the plasma, and radial transport of ions of one isotope across plasma of another isotope species. The main elements of the ISEP are: a) stripping of atoms in a thin carbon foil to produce secondary ions, b) acceleration of secondary ions, c) E//B analysis of the secondary ions in specially designed non-uniform magnetic and electric fields, d) counting of energy and mass analyzed secondary ions using detectors consisting of thin(1 <t(μm)< 7) CsI(Tl) scintillators deposited directly on miniature photo-multiplier tubes(PMT) mounted in vacuum. The ISEP has high contrast between atoms of neighbouring masses(>103 for E≈5keV, and much greater at higher energies), and high detection efficiency (0.06<ε<0.83 for atoms of 5<E(keV)<150). The ISEP detectors have very low sensitivity for neutron and gamma-ray radiation (10–7 of ion detection efficiency). This makes possible reliable use of ISEP with deuterium-tritium (D-T) plasmas in JET and ITER using only modest amount of shielding. First results of the use of ISEP on JET, demonstrating capabilities of this instrument for determination of H-D composition of plasmas, are also presented in this paper.