Depth distributions of energy dissipation and implantation ranges in the polymer resist polymethyl methacrylate (PMMA) have been calculated by using the TRIM computer code. The relevance of such calculations to experimentally obtained data in connection with ion‐beam lithography is discussed. It is concluded that the result of the computer simulations may be of value in order to explain the responsible mechanism for the modification (solubility, erosion rate) of a resist material due to ion irradiation. Experimental results of PMMA erosion during ion irradiation are also presented, showing a strong relationship between the erosion and electronic stopping.