The radiationless decay mechanism of photoexcited cytosine has been supported by exploring the important potential energy surfaces using multi-reference configuration-interaction (MRCI) ab initio methods for the gas-phase keto-tautomer. Minimum energy paths connect the Franck-Condon region to a minimum on S1 (ππ*). Two energetically accessible conical intersections with the S0 surface are shown to be connected to this minimum: one involves N3 distorting in a sofa conformation, and the other involves a twisting about the C5-C6 bond. Studies on the fluorescent cytosine analog 5-methyl-2-pyrimidinone (5M2P) reveal very similar distortions throughout the decay paths of both bases. The different photophysical behavior of the two bases is attributed to energetic differences. Vertical excitation in cytosine occurs at a higher energy, creating more vibrational energy than 5M2P, and the S1 minimum for 5M2P is too low to access an intersection with S0, causing population trapping and fluorescence. To probe the structural reasons behind the higher absorbance energy of cytosine, a series of 2-pyrimidinone derivatives with electron-donating or electron-withdrawing groups at C4 and C5 were studied with MRCI. Results reveal a predictive trend in the ππ* energies which parallels the trend in experimental values well, and they underscore the photophysical importance of the presence, position, and orientation of the C4-amino group on cytosine.