The DNA nucleoside 2'-deoxyguanosine (dG) normally pairs only to 2'-deoxycytidine (dC). However when subjected to reactive oxygen species the C8 position oxidizes converting dG into 8-oxo-2'-deoxyguanosine (OdG), which can form stable base pairs with both dC and 2'-deoxyadenosine (dA). OdG:dA mismatches are quite deleterious to cells since they can lead to dG to T mutations. Interestingly, some polymerases prefer to incorporate dCTP opposite a template OdG, while others prefer to incorporate a dATP. For example, Klenow Fragment-exo from E. coli (KF-exo) and Human DNA polymerase β (pol β) have dCTP:dATP incorporation ratios of 7 and 4, respectively, while DNA polymerase I-exo from Bacilius stearothermophilus (BF-exo) has an incorporation ratio of 0.11. In order to better understand the different nucleotide incorporation preferences of these polymerases, we studied the incorporation of dCTP and dATP opposite dG, OdG and various analogues of these nucleotides. These analogues, which differed in the imidazole ring, included 8-ChlorodG (CldG), 8-BromodG (BrdG), 8-iododG (IdG), 8-thiodG (SdG) and 9-deazadG (CdG). By testing each analogue with each polymerase, we hope to gain insight into how atoms of different sizes at C8 affect the incorporation of dCTP or dATP opposite OdG.