Aromatic onium salts are known compounds with well understood photoactivity. Their use as acid-generating species has, in the last few years, improved the performance of photoresists, particularly for microelectronics applications. There has, however, been little research dedicated to the use of these photoinitators in aqueousrdevelopable resists. This work investigates the feasibility of employing onium salts in the formulation of resists suitable for aqueous development. The ability of the onium salts to photoinitiate the cationic polymerization or cross-linking of epoxide residues was exploited using water-soluble or water-dispersible copolymers containing glycidyl methacrylate (GMA). The comonomers included N-vinyl pyrrolidinone (NVP), NN-dimethylacrylamide (DMAc) and 2-hydroxyethyI methacrylate (HEMA). Various copolymers were prepared by radical copolymerization to low conversion, typically in chloroform. Several onium salts were prepared by literature methods and their efficiency examined using a model copolymer. The photoacid generator (PAG) that performed best was then used in the examination of the aqueous systems.