The reaction of aroyl chlorides 1 with KSeCN and ethyl diazoacetate (6) in acetone at room temperature yields ethyl 2-aroyl-5-(aroylimino)-2,5-dihydro-1,2,3-selenadiazole-4-carboxylates 7 (Scheme 3). A reaction mechanism via the initial formation of the corresponding aroyl isoselenocyanates 2 followed by a 1,3-dipolar cycloaddition of the diazo compound with the C=Se bond to give ethyl 5-(aroylimino)-4,5-dihydro-1,2,3-selenadiazole-4-carboxylates of type D is proposed. Acylation of the latter at N(2) leads to the final products 7. Deacetylation of 7 to give ethyl 5-(aroylimino)-1,2,3-selenadiazole-4-carboxylates 10 is achieved by treatment of 7 with morpholine (Scheme 5). The intermediate isoselenocyanates 2 partially oligomerize to give two different oligomers. The symmetrical one reacts with morpholine to yield selenourea derivatives 12 (Scheme 6).