The gas-phase ion chemistry of protonated 0,0-diethyl 0-aryl phosphorothionates was studied with tandem mass spectrometric and ab initio theoretical methods. Collision-activated dissociation (CAD) experiments were performed for the [M + HI+ ions on a triple quadrupole mass spectrometer. Various amounts of internal energy were deposited into the ions upon CAD by variation of the collision energy and collision gas pressure. In addition to isobutane, deuterated isobutane C,D,, also was used as reagent gas in chemical ionization. The daughter ions [M + H -C,H,]" and [M + H -2C,H,]+ dominate the CAD spectra. These fragments arise via various pathways, each of which involves y-proton migration. Formation of the terminal ions [M + H -2C,H, -HzO]+, [M + H -2C,H, -H2S]+, [ZPhOH,]+, [ZPhSHz]+, and [ZPhS]+ [Z = substituentfs) on the benzene ring1 suggests that (1) the fragmenting [M + H]+ ions of 0,0-diethyl 0-aryl phosphorothionates have protons attached on the oxygen of an ethoxy group and on the oxygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl migration to sulfur occurs; (3) the fragmenting rearranged [M + HI+ ions have protons attached on the oxygen of an ethoxy group and on the sulfur of the thiophenoxy group. To get additional support for our interpretation of the mass spectrometric results, some characteristics of three protomers of 0,0-diethyl 0-phenyl phosphorothionate were investigated by carrying out ab initio molecular orbital calculations at the RHF/3-21G* level of theory. (1 Am Sot Mass Spectro~~ 2996, 7, 189-297