The threshold photoelectron spectrum (TPES) of iron pentacarbonylFe(CO)5, is obtained over an energyrange 5−35 eV using a synchrotron radiation source. Ab initiocalculations at a level of theory more refinedthan Koopmans' theorem yield ionization potentials in better agreementwith experiment and allow theassignment of the origin of most of the observed bands in the TPES onthe basis of a decomposition in(quasi) local contributions. From the measured appearancethreshold of FeC+ the bond energyD0(Fe+−C)= 84.2 ± 4.1 kcal/mol is obtained, and the enthalpyΔHf(Fe+−C) = 366.0 ±6.0 kcal/mol is derived.