Several onium salts have been synthesized and compared as cationic photoinitiators in an epoxy resist. 1-(4-Hydroxy-3-methylphenyl)tetrahydrothiophenium hexafluoroantimonate exposed at 365 nm gave the best results, due to its superior absorption at this wavelength. A series of bis-(triphenyl phosphoranylidene) ammonium salts performed poorly with 254 nm, 365 nm and E-beam irradiation even with the addition of photosensitizer. Triphenyl sulfonium hexafluoroantimonate and triphenyl sulfonium triflate formulated the most sensitive epoxy resist materials to both 254 nm and E-beam radiation sources (0.32 mJ cm(-2) and 0.3 mu C cm(-2), 59.2 mJ cm(-2) and 8.0 mu C cm(-2) at 20 kV, respectively).