The negative-tone proton exposure characteristics of a chemically amplified resist based on an epoxy resin (Shell SU-8) and an onium salt sensitizer (General Electric UVE-1014) are described. This resist was developed by IBM for UV, electrons, and x rays, for which it exhibits high contrast, good etch resistance, low minimum dose for exposure, excellent adhesion to a variety of substrates, and resolution of better than 0.1 mum. In the present work, the exposure properties of this resist were measured as a function of sensitizer concentration when exposed with 100 keV protons. The best contrast measured for 100 keV protons was 1.4, at a sensitizer concentration of 1 wt % and the corresponding minimum dose for exposure was 0.13 muC/cm2. Sub-0.1 mum resolution was also demonstrated using ion beam proximity printing. However, the resist develops as a porous mat for low ion doses, a phenomenon which can result in significant pattern edge roughness. It is suggested that this problem arises because the resist is sufficiently sensitive to register single ion tracks.