The energy distributions of ions and electrons extracted from a duoplasmatron ion source have been measured by a 127° cylinder spectrometer, providing information on the axial potential distribution in the source. The energy resolution of the spectrometer was about ±1 eV. Compared to measurements with Langmuir probes, this method has the advantage that the plasma potential is not disturbed. The energy spectrum of the extracted ions shows up to three peaks, which are related to different places of ionization in the source. The total energy spread is up to 100 eV. This energy spread can be influenced (down to 6 eV) by the potential of the expansion cup. Surprisingly, 90% of the extracted electrons are not created at the cathode but in the plasma of the intermediate electrode. This knowedge allows a better optimization of the source by independent regulation of gas pressure, intermediate electrode potential, and cathode potential. For optimum ionization of the working gas the potential of the intermediate el...