Ethoxycarbonyl isothiocyanate reacts additively with hydrazine, ethoxycarbonylhydrazine, as well as amino‐ and 1,2‐diaminoguanidine. Simultaneous or subsequent cyclisation of the resulting 1:1‐ or 2:1‐adducts in acidic or alkaline media yields substituted 1,3,4‐thiadiazoles or 1,2,4‐triazoles, respectively.