Abstract The energy distribution of ions in the plasma during unbalanced magnetron sputtering was measured with an energy-resolved mass spectrometer. Typical energy distributions of the main ion species were measured for sputtering of a Cu target in an Ar atmosphere in the pressure range from about 0.1 to 10 Pa. The effects of the discharge pressure and of the distance between target and mass spectrometer on the ion energy distribution were studied. A low energy (thermalised) peak in the ion energy distribution function was observed at an energy corresponding to the plasma potential. In addition, a high energy tail was observed at energies of about 3–30 eV above the low energy peak, not only for sputtered ionised atoms but also for the Ar ions. The ion energy distribution of this high energy tail can be fitted with an exponential function; the mean energy was about 1–5 eV. The integrated intensity of the high energy tail was a function of the pressure-distance product p × d, where d is the distance of the mass spectrometer from the magnetron. The function is close to exponential decrease with increasing p × d product for the high energy tail of the Cu + energy distribution. A similar exponential decrease was observed also for the high energy tail of the Ar + energy distribution with the exception of lowest pressures (<0.2 Pa) where the intensity of the Ar + high energy tail increases with increasing pressure. An explanation of these observations is proposed based on energy transfer from sputtered atoms to Ar gas atoms during collisions.