2-Aminopurine (2AP) is an adenine analogue that has a high fluorescence quantum yield. Its fluorescence yield decreases significantly when the base is incorporated into DNA, making it a very useful real-time probe of DNA structure. However, the basic mechanism underlying 2AP fluorescence quenching by base stacking is not well understood. A critical element in approaching this problem is obtaining an understanding of the electronic structure of the excited state. We have explored the excited state properties of 2AP and 2-amino, 9-methylpurine (2A9MP) in frozen solutions using Stark spectroscopy. The experimental data were correlated with high level ab initio (MRCI) calculations of the dipole moments, (mu) over right arrow (0) and (mu) over right arrow (1), of the ground and excited states. The magnitude and direction of the dipole moment change, Delta(mu) over right arrow (01) = (mu) over right arrow (1) - (mu) over right arrow (0), of the lowest energy optically allowed transition was determined. While other studies have reported on the magnitude of the dipole moment change, we believe that this is the first report of the direction of Delta(mu) over right arrow, a quantity that will be of great value in interpreting absorption spectral changes of the 2AP chromophore. Polarizability changes due to the transition were also obtained.