Adenine and guanine have been observed in meteor material. Because of this, they are thought to have been formed in the interstellar medium. However, while there exist several theoretical works exploring the mechanisms of formation for adenine, guanine has received only minor attention. In this work, a retrosynthetic analysis strategy was employed to suggest a new mechanism of guanine formation from species previously observed in the interstellar medium: HCN, CCO, HNCNH, and its isomer H 2 NCN. MP2/6-311++G(2d,2p) calculations were used to evaluate the Gibbs free energy of the minimum and transition states involved in a simple reaction mechanism, involving only five steps. The intermediate species are highly polar (dipole moment range of 4.35−7.72 D), which make them candidates for observation in the rotational spectra. In terms of energy, this new mechanism is very exergonic, and the energy barriers are monotonically decreasing. The value of the energy barrier for the first step in this mechanism was substantially reduced when water molecules were explicitly introduced as a way to represent water-ice-coated grain surfaces.