The rotationally resolved spectrum of the v11 (E′) fundamental band of Fe(CO)5 at 649 cm-1 has been measured in a supersonic jet using infrared diode laser absorption spectroscopy. Adopting a ground state A rotational constant of 0.030 75 cm-1, calculated from the electron diffraction structure, the following rotational constants were determined; A11 = 0.030 71(3) cm-1, B0 = 0.026 82(11) cm-1 and B11 = 0.02683(10) cm-1. The B rotational constants are in close agreement with recent ab initio calculations. The band origin is 649.078 9(22) cm-1 and the Coriolis coupling constant ζ11 = +.838 8(10). © 1999 Taylor & Francis Ltd.