Fluorescence excitation spectrum of acetyl cyanide (CH3COCN) in a supersonic jet has been obtained for the (1) (n, pi*) transition (S-1-S-0). A spectral origin band is located at 27 511 cm(-1), and its fluorescence lifetime is measured to be 3.5+/-0.3 mu s. The S-1 state vibrational modes associated with CH3 torsion, CCO bend, CCN bend, and CO wag are found to be optically active. The fluorescence intensity decreases with increasing the energy from the origin up to similar to 2000 cm(-1), while the fluorescence lifetime changes little over the same energy region. Instead, a broad background signal appears in the high energy region, indicating that the intramolecular vibrational redistribution (IVR) process becomes important in the S-1 state as the density of states increases. (C) 1999 American Institute of Physics. [S0021-9606(99)00515-2].