AbstractReaction of phenylglyoxalyl bromide arylhydrazones (III) with thiourea in ethanol produces 2‐amino‐4‐phenyl‐5‐arylazothiazoles (XI) instead of the expected 2‐benzoyl‐4‐aryl‐5‐imino‐Δ2−1,3,4‐thiadiazolines (V) obtained from III and potassium thiocyanate. Phenacyl thiocyanate (IV) couples with diazotized anilines to give V. The mechanisms of formation of V and XI from VI and III, respectively, are postulated. Nitrosation of V gives the correspondingN‐nitroso derivatives (VII), which decompose upon refluxing in xylene to give 2,4‐disubstituted Δ2−1,3,4‐thiadiazolin‐5‐ones (VIII). The thiadiazolines V give the respectiveN‐aeyl derivatives IX and X with acetic a