Interconversion of five isomeric tautomers of thioformohydroxamic acid and its aceto- and fluoro-substituted derivatives via intramolecular hydrogen transfer was investigated by ab initio theoretical calculation. The transfer potential surfaces, the global isomeric structures, and the transition geometries of intramolecular hydrogen transfer were determined at the MP2/6-31+G** level of calculation. The energy was further analyzed by a single point calculation using MP2/6-31++G**//MP2/6-31+G**. Not counting the unstable charge separating species (S3) which may be an important conformation in the formation of metal complexes, the order of stability of these tautomers was S2Z > S2E > S1Z > S1E, calculated at the MP2 level with zero point energy consideration. The S2Z and S2E are thione forms, whereas S1Z and S1E are thiol forms. The ring structure of the transition state construction dominates the energy barrier of the intramolecular hydrogen transfer, which initiates the interconversion among the five isomeric tautomers. Because the proton attached to the N-atom in the thione form is relatively easy to dissociate, we believe that thioformohydroxamic acid in the dissociating proton in the gas phase is an N-acid. However, in the thiol form it is instead a S-acid rather than an O-acid. Substitution effect (aceto and fluoro derivatives) on the stability of tautomeric structures and intramolecular hydrogen transfer barriers was also examined.