A high resolution system for the measurement of the energy loss spectra of 20–250 keV heavy ions incident on gas targets is described. The high resolution is obtained by decelerating the ion beam to a low, well defined energy before performing the energy analysis. The energy loss spectrum is determined by changing the accelerator energy to compensate for energy lost in collisions with the gas atoms. The system is arranged so that normal fluctuations in the accelerator's power supply do not affect the energy resolution. The resolving power of the system is 2.5×104. The primary proton beam has a detected WHM of 2 V at 50 kV. An energy loss spectrum is shown for which the collision‐excited 11S‐21Ptransition in helium is clearly resolved.