N1,N-6 -ethenoadenine (N1,N-6-epsilonA) and N3,N-4-ethenocytosine (N3,N-4-epsilonC) nucleos(t)ides are widely used fluorescent biochemical probes. Interestingly, the fluorescence of these compounds is highly dependent on the acidity of the medium. It is well established that the neutral form of the N1,N-6-epsilonA chromophore is responsible for its fluorescence, whereas the protonated form of N3,N-4-epsilonC is slightly fluorescent. However, the origin of the pH-dependent fluorescence has remained elusive. In the current work, the electronic excitation and emission gas-phase spectra of 3-Me-N1,N-6-epsilonA (1) and 1-Me-N3,N-4-epsilonC (2) were calculated using configuration interaction singles (CIS), time dependent density functional theory, and multiconfigurational quasidegenerate second-order perturbation theory methods. The solvatochromic shift of 1 and 2 due to hydration was computed using a stepwise molecular dynamics-semiempirical CIS method. Good agreement between the computed and experimental spectra was obtained, both in the gas phase and in aqueous solution. Our results suggest that the pH-dependent loss of fluorescence of I is due to interaction between the lowest :pipi* and npi* excited states, which may lead to rapid radiationless decay to the ground state.