The reaction of thiophosgene with α-diazocarbonyl compounds produced mixtures of 1,2,3-and 1,3,4-thiadiazoles, in which the latter predominated, irrespective of whether the bis(α-diazoacyl)mercury derivatives or the free diazo compounds were used. In contrast, ethyl chlorodithioformate gave exclusively 1,3,4-thiadiazoles with α-diazocarbonyl compounds in the presence of triethylamine and only 1,2,3-thiadiazoles when the bis(α-diazoacyl)mercury derivatives were used.