Oxidative lesions account for much of the spontaneously occurring DNA damage in normal cells and, left unrepaired, can be mutagenic or cytotoxic. We have investigated the capacity of purified human enzymes to initiate the base excision repair (BER) of oxidative lesions in model nucleosomes. In a construct where the minor groove of a thymine glycol lesion faced outward from the histone octamer, the human DNA glycosylase NTH1 (hNTH1) processed the lesion with nearly the same efficiency as in naked DNA. The hNTH1 reaction did not generate free DNA, indicating that the first step in BER occurred without irreversibly disrupting nucleosomes. Instead, lesion processing entailed the formation of nucleosome-hNTH1 ternary complexes that could be visualized in a gel mobility shift assay. These complexes contained both processed and unprocessed DNA. hNTH1 processing of lesions whose minor groove faced toward the histone octamer was poor at low hNTH1 concentrations but increased substantially as hNTH1 concentrations increased to nearly physiological levels. Additionally, an inward-facing lesion near the nucleosome edge was more efficiently processed than one closer to the nucleosome dyad. These observations suggest that access to sterically occluded lesions entails the partial, reversible unwrapping of DNA from the histone octamer, allowing hNTH1 to capture its DNA substrate when it is in an unwound state.