The cyclisation of various ortho-buta-1,3-diynylaryldiazonium salts was studied and shown to depend on the reaction conditions and nature of the substituents on the benzene ring. The reaction leads to 4-chloro-3-ethynylcinnolines, and/or 3-ethynyl-4-hydroxycinnolines, the latter undergoing subsequent cyclisation to give furo[3,2-c]cinnolines.