The syntheses of certain 4-(alkylthio)-, 4-[(arylalkyl) thio]-, and 4-(arylthio) imidazo [4, 5-d] pyridazines from imidazo [4, 5-d] pyridazine-4-thione (1, 4-S1P) are described. When 1 equiv of alkylating agent is used, alkylation occurs, as expected, on sulfur; however, when an excess of alkylating agent is employed, dialkylated products are formed. A rigorous spectroscopic study and an unequivocal synthesis of 6-methyl-4-(methylthio) imidazo-[4, ...