Time-resolved IR spectroscopy has been used to study the oxidative addition of H-2 to Fe(CO)(4) and its reverse reaction, the reductive elimination of H-2 from Fe(CO)(4)H-2, in the gas phase. The rate constant for oxidative addition of H-2 shows little temperature dependence, indicating that if there is an activation barrier for this process it is small (<4 kcal mol(-1)). The activation barrier for the reductive elimination of H-2 is 20.5 +/- 2.1 kcal mol(-1). From these measurements, the average of the dissociation energies for the two Fe-H bonds in Fe(CO)(4)H-2 is calculated to be 62 +/- 2 kcal mol(-1). Kinetic measurements employing D-2 indicate small kinetic isotope effects for both the oxidative addition and reductive elimination reactions.