We have explored the potential energy surface for the dissociation of the pyridazine molecular ion using G3 model calculations. The pathways have been obtained for the formation of five possible $C_4H_4^{+{ullet}}$ isomers by the loss of $N_2$ and the consecutive $H^{ullet}$ loss. It is predicted that the methylenecyclopropene radical cation is the predominant product in the loss of $N_2$, which is formed via the allenylcarbene radical cation, and $CH_2=C-C{equiv}CH^+$ is the predominant product in the consecutive $H^{ullet}$ loss.