The reaction kinetics of electron impact ionization generated FeCO+ cations with Oz is investigated using a triple-cell Fourier Transform Ion Cyclotron Resonance spectrometer. Two reaction channels are observed: (1) exothermic formation of FeO+ with oxidation of the CO ligand; (2) slightly endothermic ligand substitution giving FeO2+. Residual water also reacts on FeCO+ and FeO2+. Varying the relaxation delay of ions before reaction allowed us to study the radiative relaxation of FeCO+. Three groups of differently excited states of FeCO+, termed as (FeCO+)**, (FeCO+)* and (FeCO+)(0) by order of decreasing internal energy, can be differentiated according to their different reactivities. While (FeCO+)(**) and (FeCO+)(*) lead to FeO2+, only (FeCO+)(0) gives FeO+.