AbstractMeasurements of the total and selected partial cross sections for the electron impact ionization of cyclopentadienyltrimethylplatinium (C5H5)Pt(CH3)3[CpPtMe3], bismethylcyclopentadienylruthenium (CH3C5H4)2Ru [(MeCp)2Ru], and bismethylcyclopentadienylferrum (CH3C5H4)2Fe [(MeCp)2Fe] in the energy range from threshold up to 90 eV are presented. A double focussing mass spectrometer with an improved ion extraction system to avoid the discrimination of fragment ions with excess energy was used. The following values for the total single ionization cross sections σ at an impact energy of 70 eV and for the ionization energies (EI) were determined: σ = (13.7 ± 2.5) 10−16cm2,EI= 7.6 ± 0.5 eV [CpPtMe3], σ = (23.2 ± 4.2) × 10−16cm2,EI= 7.5 ± 0.5 eV [(MeCp)2Ru], σ = (20.2 ± 3.6) × 10−16cm2,EI= 8.1 ± 0.5 eV [(MeCp)2Fe]. The results are compared with known mass spectra of the Pt‐ and Fe‐compound. Schemes for the fragmentation and ion formation processes in the ion source are discussed. The fragment ion composition as a function of the ion source temperature was investigated. The production rate of pure metal ions is small. The measured total single ionization cross sections when compared with calculations using two additivity rules were found to be lower than