Methylisothiazoles undergo phototransposition in neutral solution to m
ethylthiazoles by a single permutation process. Methylisothiazole -->
methylisothiazole transpositions, previously reported to occur, were n
ot detected in these reactions. In trifluoroacetic acid solvent, proto
nated methylisothiazoles and methylthiazoles phototranspose by P4 and
P5 permutation pathways, respectively, resulting in a unique phototran
sposition cycle.