The photochemistry of Fe(CO)5 (5) has been studied in heptane, supercritical (sc) Ar, scXe, and scCH(4) using time-resolved infrared spectroscopy (TRIR). Fe-3(CO)(4) ((3)4) and Fe(CO)(3)(solvent) (3) are formed as primary photoproducts within the first few picoseconds. Complex 3 is formed via a single-photon process. In heptane, scCH(4), and scXe, (3)4 decays to form (1)4-L (L = heptane, CH4, or Xe) as well as reacting with 5 to form Fe-2(CO)(9). In heptane, 3 reacts with CO to form (1)4(.)L. The conversion of (3)4 to (1)4(.)L has been monitored directly for the first time (L = heptane, k(obs) = 7.8(+/-0.3) x 10(7) s(-1); scCH(4), 5(+/-1) x 10(6) s(-1); scXe, 2.1(+/-0.1) x 10(7) s(-1)). In scAr, (3)4 and 3 react with CO to form 5 and (3)4, respectively. We have determined the rate constant (k(CO) = 1.2 x 10(7) dm(3) mol(-1) s(-1)) for the reaction of (3)4 with CO in scAr, and this is very similar to the value obtained previously in the gas phase. Doping the scAr with either Xe or CH4 resulted in (3)4 reacting with Xe or CH4 to form (1)4(.)Xe or (1)4(.)CH(4). The relative yield, [3 4]:[3] decreases in the order heptane > scXe > scCH(4) much greater than scAr, and pressure-dependent measurements in scAr and scCH(4) indicate an influence of the solvent density on this ratio.