Electrospray ionisation mass spectrometry (ESI-MS) has been used to characterise alkali metal and alkaline earth metal complexes formed in methanol solutions containing one of several synthetic cryptand ligands, or the naturally occurring ionophore valinomycin. The spectra are very simple and easily interpreted, reflecting the gentle nature of the electrospray ionisation process. Mass spectra were also obtained for solutions containing equimolar concentrations of each of the alkali metal ions or alkaline earths, with valinomycin or the cryptands. In each case the intensities of peaks due to metal complexes correlated well with the expected composition of the solution predicted from stability constants obtained by other techniques. These results suggest that ESI-MS will not only prove to be of great benefit for assisting determination of the composition of metal complexes formed in systems of these types, but will also be valuable as a tool for rapidly establishing the metal ion selectivity of new ligands.