Of primary concernwiththeintegration of Hall thrusterson conventional satellitedesigns isthepossibledamaging effect of high-energy exhaust ions impinging upon spacecraft surfaces. This paper reports on measurements of plasma ion-energy distributions within the plume of an SPT-100 Hall thruster using a custom-designed molecularbeam mass spectrometer. With this instrument ion energy was measured over a complete 360-deg circumference about the thruster at a radius of 0.5 m from the exit plane and over a total inclusive arc of 260 deg at 1.0-m radius. These data uncovered the existence of high-energy ions departing the thruster at angles exceeding 90 deg from the thrust vector and continuing well into the backe ow region of the plume. Through an analysis of the energy structure, the evidence of charge-exchange collisions occurring between plume ions and background neutrals was documented; such collisions produced anomalous distributions of ions having voltages greater than that applied to the thruster discharge. Nomenclature Ac = area of collector, m 2 d = analyzer plate separation, m Ei = ion energy, J or eV e = elementary charge, C f (ui) = ion velocity distribution, s/m f (Vi) = ion voltage distribution, s/m GCEM = gain of electron multiplier K45 = spectrometer constant k = Boltzmann constant, J/K L = analyzer interslit distance, m mi = mass of ion, kg ni = ion density, m i 3 q = ion integer charge state r = radial distance from thruster, m ui = ion velocity, m/s Vb = main distribution voltage, V Vi = ion voltage, V Vm = most-probable voltage, V Vp = repelling plate voltage, V w = analyzer slit width, m x = spatial coordinate, m y = spatial coordinate, m h = angle, deg s i = distribution half-width at e i 1 point, V