The potential energy surface (PES) for the formation of C4H4 center dot+ from the pyridine radical cation by loss of HCN was determined from quantum chemical calculations using the G3//B3LYP method. The complete reaction pathways for the formation of the low-energy C4H4 center dot+ isomers, radical cations of methylenecyclopropene (MCP center dot+), vinylacteylene (VA(center dot+)), cyclobutadiene, and butatriene were obtained. Based on the PESs, a Rice-Ramsperger-Kassel-Marcus model calculation was performed to investigate the dissociation kinetics. The calculated dissociation rate constants agreed with the previous experimental data. It was predicted that a mixture of MCP center dot+ and VA(center dot+) was formed by loss of HCN. The formation of MCP center dot+ was more favored near the dissociation threshold and at high energies, whereas the formation of VA(center dot+) was more favored at the low energies corresponding to the ion lifetime of microseconds.