N-Arylamino-1,3-diaza-1,3-butadienes 4 are shown to undergo regioselective reactions with phenyl- and chloroketenes resulting in high yields of 3-aryl-2-methylthio-6-phenyl-4(3H)-pyrimidinones 7. Similar reactions with bromo- and iodoketenes, resulted, via aziridinium intermediates 12, in good yields of 3-aryl-5-(N-arylamino)-2-methylthio-6-phenyl-4(3H)-pyrimidinones 13. The mechanistic aspects of cycloadditions and semi-empirical AM1 calculations for these diazabutadienes are also reported.