The S-0 -> S-1 vibronic spectrum and S-1 state nonradiative relaxation of jet. cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(-1) resolution. The 0(0)(0) rotational band contour is polarized in-plane, implying that the electronic transition is (1)pi pi*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the (1)pi pi* (S-1) transition of 5FCyt. The spectral region from 0 to 300 cm(-1) is dominated by overtone and combination bands of the out-of-plane nu'(1) (boat), nu'(2) (butterfly), and nu'(3) (HN-C6H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the (1)pi pi* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm(-1); this is attributed to the 1 pi pi* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the nu'(1), nu'(2), and nu'(3) coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm(-1), rapid nonradiative relaxation (k(nr) >= 10(12) s(-1)) sets in, which we interpret as the height of the (1)pi pi* state barrier in front of the lowest S-1/S-0 conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the nu' = 0, 2 nu'(1), 2 nu'(2), 2 nu'(1) + 2 nu'(2), 4 nu'(2) and 2 nu'(1) + 4 nu'(2) levels are determined from Lorentzian widths fitted to the rotational band contours and are tau >= 75 ps for nu' = 0, decreasing to tau >= 55 ps at the 2 nu'(1) + 4 nu'(2) level at +234 cm(-1). These gas-phase lifetimes are twice those of S-1 state cytosine and 10-100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its S-1 ((1)pi pi*) state is essentially controlled by the same similar to 1200 cm(-1) barrier and conical intersection both in the gas phase and in solution.