The cavity ringdown absorption spectrum of 4-cyclopentene-1,3-dione was recorded near 487 nm in a room-temperature gas cell. The very weak band system (epsilon approximate to 0.05 dm(3) mol(-1) cm(-1)) in this region is due to the T-1(n,pi*) <- S-0 electronic transition. The origin-band maximum was observed at 20 540.0 +/- 0.5 cm(-1). We have assigned about 40 vibronically resolved bands in a region extending to +1100 cm(-1) relative to the origin. Assignments were aided by quantum-chemical calculations of the T-1 <- S-0 0-0 excitation energy as well as ground-state vibrational frequencies. From the CRD spectral assignments, we determined fundamental frequencies for several vibrational modes in the T-1 excited state, including the lowest-energy ring-bending and -twisting modes, nu'(19) (b(1)) and nu'(14) (a(2)), respectively. Their fundamentals in the T, state are 160.5 and 246 cm(-1), compared to 99 and 239 cm(-1), respectively, in the So ground state. The increases in these ring frequencies upon electronic excitation signify that the nominal n -> pi* chromophore is delocalized to include the conjugated ring atoms. The extent of this delocalization is different in the T-1(n,pi*) vs S-1(n,pi*) excited states. This conclusion is based on observed differences in T-1 vs S-1 ring fundamental frequencies.