Distinct photophysical behavior of nucleobase adenine and its constitutional isomer, 2-aminopurine, has been studied by using quantum chemical methods, in particular an accurate ab initio multiconfigurational second-order perturbation theory. After light irradiation, the efficient, ultrafast energy dissipation observed for nonfluorescent 9H-adenine is explained here by the nonradiative internal conversion process taking place along a barrierless reaction path from the initially populated ¹ (ππ* L _a ) excited state toward a low-lying conical intersection (CI) connected with the ground state. In contrast, the strong fluorescence recorded for 2-aminopurine at 4.0 eV with large decay lifetime is interpreted by the presence of a minimum in the ¹ (ππ* L _a ) hypersurface lying below the lowest CI and the subsequent potential energy barrier required to reach the funnel to the ground state. Secondary deactivation channels were found in the two systems related to additional CIs involving the ¹ (ππ* L _b ) and ¹ ( n π*) states. Although in 9H-adenine a population switch between both states is proposed, in 7H-adenine this may be perturbed by a relatively larger barrier to access the ¹ ( n π*) state, and, therefore, the ¹ (ππ* L _b ) state becomes responsible for the weak fluorescence measured in aqueous adenine at ≈4.5 eV. In contrast to previous models that explained fluorescence quenching in adenine, unlike in 2-aminopurine, on the basis of the vibronic coupling of the nearby ¹ (ππ*) and ¹ ( n π*) states, the present results indicate that the ¹ ( n π*) state does not contribute to the leading photophysical event and establish the prevalence of a model based on the CI concept in modern photochemistry.