Electrophilic substitution of diazo compounds with cyclopropenylium and other Hückel‐aromatic cations affords products of versatile reactivity: e.g (1), R1 = R2 = Ph, isomerizes to the pyridazine 2; on irradiation 1 forms the carbene. (Figure Presented.) Copyright © 1979 by Verlag Chemie, GmbH, Germany