ψ- exo- trans-Dienoliron tricarbonyl complexes yield syn, syn-dienyliron tricarbonyl cations and ψ- endo- trans-dienoliron tricarbonyl complexes yield syn, anti-dienyliron tricarbonyl cations with ⩾ 97% stereospecificity in fluorosulfonic acid at −60°C and with lower stereospecificity of 0°C. Aqueous or methanol quenching of these cations at 0°C or below proceeds in the above sense with ⩾97% stereospecificity. However, aqueous quenching of syn, anti cations at 23°C produces up to ca. 30% ψ- exo alcohol which is not the result of acid catalyzed isomerization of ψ- endo alcohol subsequent to quenching. Sodium borohydride reduction of tricarbonyl( cis, trans-4-methyl-3,5-heptadienone)iron(V) gives a single cis-dienol complex which ionizes non-stereospecifically under all conditions. syn, anti-Tricarbonyl(4-methyl-3,5-heptadien-2-yl)iron cation (IV) is configurationally stable in FSO 3H and H 2SO 4 for hours at room temperature. The rate of slow isomerization of IV to its syn, syn-isomer is independent of bisulfate ion concentration and is unaccompanied by deuterium incorporation in D 2SO 4.