Gas phase protonation of ferrocene has been studied experimentally in four different reaction systems, using high energy Collision Induced Decomposition mass spectrometry to follow the progress of reaction at different temperatures and pressures. Experimental evidence is obtained for the existence of three stable isomers as predicted theoretically. The most stable form is shown to be most probably metal-protonated, but the energies of the metal-protonated and agostic forms are within 10 kJ mol -1 of each other. The ring-protonated isomer is higher in energy by ≤ 62 kJ mol -1 . Intramolecular isomerization could not be detected, indicating relatively high energy or entropy barriers separating the isomers. The intermediate complex formed by exo attack of t-C 4 H 9 + on ferrocene was also studied and observed to change structure, from a σ-to a π-complex, leading to the population of different protonated isomers of ferrocene, depending on the temperature.