The neutral, positive, and negative beam components have been measured at the exit of the gas neutralizer of a powerful D−/H−neutral beam injector. Their relative fractions have been studied as a function of the neutralizer gas target thickness. These experiments used a diagnostic consisting of a large electrostatic deflector and three calorimetric targets, one for each beam component. These measurements have been performed for the first time with high power D−and H−beams (up to 200 kW, multisecond pulses). The negative ion beam energy was varied from 25 to 100 keV/nucleon. The results are in agreement with the theoretical predictions derived from the cross‐sections data. They confirmed in particular, that negative ion neutralization efficiencies of about 60% can be obtained at 100 keV/nucleon.