Abstract6‐Arylpyridazin‐3(2H)‐thiones exist mainly, if not entirely, in the thione form (1). This was shown by spectroscopic studies on some of these compounds and their substitution products either on the nitrogen or on the sulphur [in the mercaptopyridazine form (2)]. However, the reacting tautomer of these compounds was found to be dependent upon the basicity of the reaction medium. Selected 6‐arylpyridazin‐3(2H)‐thione derivatives react with formaldehyde and sec‐amines to give N‐2‐aminomethyl derivatives (8) [thione form (1)]. On the other hand oxidation and alkylation reactions gave disulphides (10) and S‐alkyl derivatives (7)