Near-UV photolysis of a mixture of Fe(CO)(5) and C3H6 in the gas phase produces the (pi-allyl) metal hydride complex, HFe(CO)(3)(eta(3)-C3H5), an intermediate implicated in iron carbonyl-catalyzed olefin isomerization. The formation of HFe(CO)(3)(C3H5) is rate limited by the addition of C3H6 to Fe(CO)(3) which has a rate constant of (2.6 +/- 0.3) x 10(-10) cm(3) molecules(-1) s(-1) at 296 K. Subsequent to propene addition, the unimolecular rearrangement of Fe(CO)(3)(eta(2)-C3H6) -->k1 HFe(CO)(3)(eta(3)-C3H5) takes place with a lower bound for k(1) of 10(10) s(-1). With the assumption of a preexponential for k(1) of 10(13) s(-1), the activation enthalpy associated with k(1) is <3.5 kcal mol(-1). The data are consistent with the establishment of an equilibrium between HFe(CO)(3)(eta(3)-C3H5) and Fe(CO)(3)(eta(2)-C3H6) with an equilibrium constant of 2.4 x 10(-5) and an isotope effect of K-eq(H)/K-eq(D) = 0.45 at 296 K. Fe(CO)(3)(C3H6)(2) forms by addition of propene to Fe(CO)(3)(eta(2)-C3H6), which is in equilibrium with HFe(CO)(3)(eta(3)-C3H5), with a phenomenological rate constant K(eq)k(+) = (4.5 +/- 0.1) x 10(-16) cm(3) molecules(-1) s(-1). A van’t Hoff plot gives Delta H = 7.2 +/- 0.6 kcal mol(-1) and Delta S = 3 +/- 2 cal K-1 mol(-1) for the process HFe(CO)(3)(eta(3)-C3H5) --> Fe(CO)(3)(eta(2)-C3H6), assuming that the rate constant for addition of C3H6 to Fe(CO)(3)(C3H6), k(+), is similar to 6 x 10(-11) Cm-3 molecules(-1) s(-1). These results can be quantitatively related to a catalytic cycle for olefin isomerization.