Liquid secondary ionization mass spectra (LSIMS) of methylsulfate salts of phenyl, isopropyl, and methyl 10-methyl-acridinium-9-carboxylate were examined under chemiluminescent conditions. Under chemiluminescence conditions, oxygenated acrydinium ions [M + 16]⁺ were observed, whose structures were elucidated as 9-oxy derivatives on the basis of their fragmentation processes obtained by linked scanning at constant B/E. The oxygenated ions are probably derived from the mass spectrometric reaction of the hydroperoxy intermediates in the chemiluminescence proposed by McCapra.