The superacid-promoted reactions of vinyl-substituted N-heterocycles have been studied. Diprotonated pyrimidines, quinoxalines, and quinazolines exhibit an unusual regioelectronic effect that controls the type of addition reaction observed. Depending on the ring position of the vinyl substituent, either conjugate addition or Markovnikov addition occurs. The mode of addition has been shown to correlate well to NBO calculated charges.