The gas-phase reactions between ethylenediamine (en) and Cu(+) have been investigated by means of mass spectrometry techniques. The MIKE spectrum reveals that the adduct ions [Cu(+)(H(2)NCH(2)CH(2)NH(2))] spontaneously decompose by loosing H(2), NH(3) and HCu, the loss of hydrogen being clearly dominant. The spectra of the fully C-deuterated species show the loss of HD, NH(3) and CuD but no losses of H(2), D(2), NH(2)D, NHD(2), ND(3) or CuH are observed. This clearly excludes hydrogen exchange between the methylene and the amino groups as possible mechanisms for the loss of ammonia. Conversely, methylene hydrogen atoms are clearly involved in the loss of molecular hydrogen. The structures and bonding characteristics of the Cu(+)(en) complexes as well as the different stationary points of the corresponding potential energy surface (PES) have been theoretically studied by DFT calculations carried out at B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level. Based on the topology of this PES the most plausible mechanisms for the aforementioned unimolecular fragmentations are proposed. Our theoretical estimates indicate that Cu(+) strongly binds to en, by forming a chelated structure in which Cu(+) is bridging between both amino groups. The binding energy is quite high (84 kcal mol(-1)), but also the products of the unimolecular decomposition of Cu(+)(en) complexes are strongly bound Cu(+)-complexes.