The addition products of dialkyldithiophosphoric acids to the C≡N bond in 3-thiocyanatophenothiazine rearrange to give relatively more stable S-(3-phenothiazinyl)-N-dialkoxythiophosphoryldithiocarbamates. An analogous derivative, formed in the reaction of diethyldithiophosphoric acid with 3-thiocyanato-10-ethylphenothiazine, is unstable and decomposes to give 3-mercapto-10-ethylphenothiazine and diethylisothiocyanatothiophosphate. The main pathway in the reaction of dialkyldithiophosphoric acids with 10-(thiocyanatoacetyl)phenothiazine appears to be acid-catalyzed cyclization of the latter to give 4-(10-phenothiazinyl)-3-thiazolin-2-one. However, the formation of 3-dialkoxythiophosphoryl-4-thiazolidinone-2-thiones is also observed in these reactions; these products result from rearrangement of the addition products of dithio acids to C≡N bonds, followed by subsequent heterocyclization via elimination of phenothiazine. The molecular structure of 3-diisopropoxythiophosphoryl-4-thiazolidinone-2-thione was investigated by x-ray structural analysis; the heterocyclic fragment in this molecule exists in an envelope conformation with the nitrogen atom deviating from the plane of the other four atoms.