The reactions between I- and the cations [(η5-dienyl)Fe(CO)3)]+ (dienyl = C6H7, 2-MeOC6H6, or C7H9) provide a rare example in which attack occurs at each of the hydrocarbon, carbonyl and metal centres. In the case of the C6H7 and 2-MeOC6H6 complexes ring adduct formation predominates, whereas with the more sterically crowded C7H9 substrate the dominant pathway is CO ligand substitution. A minor organometallic product formed in CH3NO2 and acetone solvents is believed to be the novel acyl iodide adduct [(η5-dienyl)Fe(CO)2(COI)]. However, in CH3CN this latter species is not observed. Concomitant formation of molecular iodine occurs, which is converted to I3- in the presence of the large excess of I- employed. This suggests that a single electron transfer (SET) mechanism is operating, involving initial transfer of one electron from the I- to the dienyl cations and the formation of iodine radicals. Kinetic studies in various solvents, showing the general rate law Rate = k[Fe][I-], are consistent with rate-determining initial electron transfer followed by rapid reaction between the radicals formed to give the observed products.