Photolysis of Fe(CO)s in cyclohexane at 337 nm in the presence of PR3 (R = Et, Me, n-Bu, Ph) pyridine, CNC-(CH3)3, and CH3CN was performed under conditions where secondary photolysis of photoproducts was negligible. Depending on the dispersed ligand, both 1 and 2 (Fe(CO)4L and Fe(CO)3L2, respectively) were formed. At low concentrations of dispersed ligand relative to Fe(CO)5, we report the first observation of quantum yields greater than unity for noncatalytic reactions leading to the formation of 1 and 2. The mechanism of reaction is proposed to involve the formation of multinuclear species via the reaction of Fe(CO)5 with coordinatively-unsaturated intermediates. At high ligand concentrations, the formation of multinuclear species is inhibited and the total quantum yield is 0.8. When investigated for the specific case of PEt3, the product ratio of 2/1 did not change for 1-10% conversion of Fe(CO)5. It is concluded that, at high concentrations of dispersed ligand, a single-photon process produces 2. The role of triplet intermediates was probed using triplet sensitizers and quenchers. The product ratio was unchanged when Fe(CO)5 was triplet-sensitized with xanthone in the presence of PEt3. Disubstituted product was formed when Fe(CO)4PEt3 was triplet-sensitized with benzophenone in the presence of PEt3. Finally, the distribution of products was changed with the addition of triplet quenchers. It is proposed that Fe(CO)5 photolysis produces a triplet Fe(CO)4. Further reaction with L forms triplet Fe(CO)4L, which dissociates CO in competition with intersystem crossing to the ground state.