Mass spectrometry measurements reveal that the reactions between guanidine and Cu+ in the gas phase leads predominantly to the loss of ammonia. The corresponding mechanisms were investigated by means of DFT calculations at the B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level. The attachment of Cu+ takes place preferentially at the imino nitrogen, while attachment at the amino leads to a local minimum which lies almost 26 kcal/mol higher in energy. The estimated guanidine-Cu+ binding energy is 77.9 kcal/mol. Hence, guanidine is predicted to be a stronger base than ammonia when the reference acid is Cu+, although this basicity difference is about 11 kcal/mol smaller than that found when the reference acid is H+. Two possible reaction channels, with almost equal activation barriers, lead to the loss of NH3. One of them yields the H2N-CN-Cu+ complex while the other yields the HNCNHCu+ complex. The structures and bonding characteristics of both products are discussed. Their estimated heats of formation are 225 +/- 2 and 237 +/- 2 kcal/mol, respectively. The structures and the relative stabilities of the complexes which can be formed by the solvation of these two products by ammonia have been also investigated since they are essential to explain the unimolecular NH3-loss processes observed.