Electrospray (ES) mass spectrometry provides a convenient method for the characterisation of a wide range of π-hydrocarbon complex salts of the general types [FeCp(η-arene)]BF 4 , [M(CO) 3 (η-C 7 H 7 )BF 4 (M = Cr, Mo, W), [Fe(CO) 2 L (η 5 -dienyl)]BF 4 (L = CO or PPh 3 ; dienyl = C 6 H 7 , 2-MeOC 6 H 6 , or C 7 H 9 ), [CpFe(CO) 3 ]PF 6 and [CpFe(CO) 2 (η-C 2 H 4 )]BF 4 . At low skimmer voltages (20 V), principal (molecular) ions are generally the only observed species. For the arene complexes, [FeCp(η-arene)]BF 4 , high skimmer voltages (80-135 V) are required before fragmentation is observed. However, for the carbonyl containing cations, moderate skimmer voltages (40-55 V) also give rise to [M-CO] + , [M-2CO] + ions. Very similar behaviour is observed with phosphonium and imidazolium adducts of the type [Fe(CO) 3 (η 4 -diene.Nuc)] + (diene = C 6 H 7 or C 7 H 9 ; Nuc = PPh 3 or Im) and [CpFe(CO) 2 (η 1 -C 2 H 4 . PPh 3 )] + . Comparison with fast atom bombardment (FAB) and field desorption (FD) mass spectra for the same complex salts indicates that fragmentation decreases along the series FAB > ES > FD, and that ES mass spectrometry is the most convenient and informative of the three soft ionization techniques.