Abstract The 4He + beam from a Van de Graaff accelerator is often accompanied by an 16O + beam of the same energy. If, after acceleration and before magnetic analysis, one electron is stripped from the oxygen ion to form 16O 2+, these ions will not be separated from 4He + during magnetic analysis. The 16O 2+ fraction in the 4He + beam was measured by analyzing back-scattering spectra for a thin Au film and the O to He ion charge ratio of 2:1 was confirmed by electrostatic deflection. Ion source conditions and the pressure in the beam lines strongly affect the 16O 2+ fraction and the dependence of the O 2+/O + ratio on pressure is found to be in approximate agreement with predicted values. A formula for estimating the 16O 2+ beam intensity, based on charge exchange data and measurement of the primary 16O + beam intensity, is N[ O 2+] = 13 P( torr) l( cm) N[ O +], where P is the total gas pressure, l the length of the beam line between the accelerator and switching magnet and N[O 2+] and N[O +] are the beam intensities of the respective species. The 16O 2+ contamination of 4He + beams is easily removed by electrostatic separation.