The ion‐beam sensitivity of polymer resists, mainly PMMA, was investigated for argon, gallium, helium, and hydrogen ions. The resists were exposed in a standard ion accelerator at energies of 40 to 120 keV. The samples were irradiated with doses varying between 5×1010 and 5×1014 cm−2. It was found that the resists are about a factor of 100 more sensitive for hydrogen ions than for electrons of 20 keV; whereas for helium, up to a factor of 150, and for argon and gallium a factor of more than 300 were found for positive resists. Using an energy of 120 keV, layers of more than 1.3 μm thickness can be exposed by hydrogen ions, 1.0 μm thickness by helium ions and 0.25 μm thickness by argon ions. The effective sensitivity is energy‐dependent, and within the region of 40 to 120 keV between a factor of 1.4 and 2.0, with the higher sensitivity at lower energies. The achievable resolution of structures using ion beams is very high. Resists with a sensitivity of 5×10−7 C/cm2 easily yield a resolution of 0.25 μm wide and 1.3 μm thick lines.