Thermocatalytic decomposition of diazo esters in the presence of 3-aryl-2H-azirines gives rise to azirinium ylides. The latter preferentially transform via isomerization into 2-azabuta-1,3-diene derivatives or, with excess diazo compound, via reaction with the Rh-carbenoid to form 3,4-dihydro-2H- pyrrole derivatives. In contrast, ylides generated from 2-monosubstituted or 2,2-disubstituted 3-phenyl-2H-azirines transform exclusively via isomerization into the corresponding 2-azabuta-1,3-dienes in high yields.