DNA polymerase makes errors by misincorporating natural DNA bases and base analogs. Because of the wide variety of possible mismatches and the varying efficiency with which they are repaired, structural studies are necessary to understand in detail how these mispairs differ and can be distinguished from standard Watson-Crick base pairs. 2-Aminopurine (AP) is a highly mutagenic base analog. The objective of this study was to determine the geometry of the AP x C mispair in DNA at neutral pH. Although several studies have focused on the AP x C mispair in DNA, there is not as of yet consensus on its structure. At least four models have been proposed for this mispair. Through the use of NMR spectroscopy with selective 15N-labeling of exocyclic amino nitrogens on bases of interest, we are able to resolve ambiguities in previous studies. We find here that, in two different DNA sequences, the AP x C mispair at neutral and high pH is in a wobble geometry. The structure and stability of this base mispair is dependent upon the local base sequence.