Excited-state quantum chemical calculations of two 2-alkyloamino-6-methyl- 4-nitropyridine N-oxides are presented. Several different calculation methods and different basis sets were used, which all lead to similar results, although the precise values of excited-state energies and excited-state dipóle moments differ. All methods used predict that in the Si excited state four types of isomers occur. In three cases, these excited-state local energy minima correspond to ground-state isomers, and these all have a ππ* character. The fourth excited-state minimum, which we denote L*, does not have a corresponding ground-state isomer and has an nπ* character. This isomer is stable and plays an important role in understanding the photophysics of these molecules. In addition, we also calculated barriers between these excited-state minima, using predescribed reaction pathways. The theoretical results derived in this Article are confronted with experimental data from earlier papers. © 2009 American Chemical Society.