The electrochemical oxidation of 2-phenyl-1,2-benzisoselenazol-3(2H)-one and several of its derivatives was investigated in dried acetonitrile by conventional and microelectrode techniques. The unsubstituted benzisoselenazolone (1a) as well as its derivatives substituted in position 4’ by methyl (1b) and methoxy (1c) groups are oxidized by controlled potential electrolysis into their selenoxides (2a-c). The intermediary cation radicals (CRs), eventually stabilized in the medium by addition of a water scavenger (trifluoroacetic anhydride, 1%), were detected by high-speed cyclic voltammetry at potential scan rates up to 30 kV s(-1). The corresponding reaction mechanism is of an ECE-type, in which the chemical step consists of the hydrolysis of the formed CR by residual water. A comparison with its sulphur analogues, 2-phenyl-1,2-benzisothiazol-3(2H)-ones (3), showed that the selenium CRs are three orders of magnitude more reactive towards residual water then their sulphur analogues. Similar studies of benzisoselenazolones substituted on the N-aryl ring by-electro-donating groups, 2-(3,4-dimethoxyphenyl)- (1d), 2-(3,4-methylenedioxyphenyl)- (1e) and 2-(4-dimethylaminophenyl)-1,2-benzisoselenazolone (1f), gave rise to no selenoxide formation, although intermediary CRs were detected at potential scan rates below 30 V s(-1).