In the standard electrospray ionization mass spectra of many common, low molecular mass organic compounds dissolved in methanol, peaks corresponding to ions with formula 3M + Met (M = organic molecule, Met = bivalent metal cation) are observed, sometimes with significant abundances. The most common are ions containing Mg(2+), Ca(2+) and Fe(2+). Their presence can be easily rationalized on the basis of typical organic reaction work-up procedures. The formation of 3M + Met ions has been studied using N-FMOC-proline methyl ester as a model organic ligand and Mg(2+), Ca(2+), Sr(2+), Ba(2+), Fe(2+), Ni(2+), Mn(2+), Co(2+) and Zn(2+) chlorides or acetates as the sources of bivalent cation. It was found that all ions studied form 3M + Met complexes with N-FMOC-proline methyl ester, some of them at very low concentrations. Transition metal cations generally show higher complexation activity in comparison with alkaline earth metal cations. They are also more specific in the formation of 3M + Met complexes. In the case of alkaline earth metal cations 2M + Met and 4M + Met complex ions are also observed. It has been found that 3M + Met complex ions undergo specific fragmentation at relatively low energy, yielding fluorenylmethyl cation as a major product. M + Na ions are much more stable and their fragmentation is not as specific.