We have measured the mass- and tautomerspecific S-0 -> S-1 vibronic spectra and S-1 state lifetimes of the keto-amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm(-1) resolution. The rotational contours of the 0(0)(0) band and nine vibronic bands up to +437 cm(-1) are polarized in the pyrimidinone plane, proving that the electronic excitation is to a (1)pi pi* state. All vibronic excitations up to +437 cm(-1) are overtone and combination bands of the low-frequency out-of-plane v'(1) (butterfly), v'(2) (boat), and v'(3) (H-N-C-6-H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and (1)pi pi* states are in good agreement with the experimental R2PI spectrum, but only if the calculated v'(1) and v'(2) frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the v'(1) and v'(2) overtone vibronic excitations, this implies that the (1)pi pi* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The (1)pi pi* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0(0)(0) band it is tau = 44 ps, remains at tau = 35-45 Ps up to +205 cm(-1), and decreases to 25-30 ps up to +437 cm(-1). These lifetimes are 20-40 times longer than the 0.5-1.5 ps lifetimes previously measured with femtosecond pump-probe techniques at higher vibrational energies (1500-3800 cm(-1)). Thus, the nonradiative relaxation rate of keto-amino cytosine close to the (1)pi pi* state minimum is k(nr) similar to 2.5 X 10(10) s(-1), much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm(-1) excess energy. Since high overtone bands of v'(1) and v'(2) are observed in the R2PI spectrum but only a single weak 2v'(3) band, we propose that v'(3) is a promoting mode for nonradiative decay, consistent with the observation that the v'(3) normal-mode eigenvector points toward the "C-6-puckered" conical intersection geometry.