Phenylthiazoles 1-3 and phenylisothiazoles 4-6 undergo phototransposition in benzene solvent mainly by P5, P6, and P7 permutation pathways. Phenylisothiazoles 5 and 6 also transpose via a P4 permutation process to yield phenylthiazoles 2 and 3 in less than 1% yield. In benzene saturated with D2O, 2-phenylthiazole(1) and 5-phenylisothiazole (6) each phototranspose to yield 4-deuterio-3-phenylisothiazole (4-D-4) and 4-phenylthiazole (2) without deuterium incorporation. Irradiation of 4-phenylthiazole (2) under these conditions results in rapid photodeuteration to yield 2-deuterio-4-phenylthiazole (2-D-2), which subsequently phototransposes to 5-deuterio-3-phenylisothiazole (5-D-3). These experimental results can be rationalized by a mechanism involving initial electrocyclic ring closure and sigmatropic shift of sulfur around the four sides of the azetine ring. Rearomatization of each bicyclic intermediate thus allows sulfur to insert into each position in the carbon-nitrogen sequence. As a consequence, these compounds divide into a tetrad in which isomers 1, 2, 4, and 6 interconvert mainly via Ps, P6, and P7 pathways and a dyad of two compounds in which 3 phototransposes to 5 via P5 and P7 pathways. Within the tetrad, BC-6, the bicyclic intermediate derived from 5-phenylisothiazoles (6), is postulated to undergo deuteration with simultaneous sigmatropic shift of sulfur when the reaction is carried out in benzene-D2O. This mechanistic view provides one coherent interpretation for the observed phototransposition and photodeuteration reactions.