High resolution x‐ray photoelectron spectroscopy (XPS) has been used to study the interfacial chemistry between vacuum‐deposited silver and poly(ethylene terephthalate) (PET) surfaces. The initial stages of metallization in the subatomic layer regime (∼0.1–1.0 atomic layer of Ag) were monitored. For the untreated PET, the XPS results suggest charge transfer between silver and the carbonyl oxygen in PET in the early stages of metallization. The effect on the untreated PET is small with only about 7% of the PET surface atoms providing reaction sites. Plasma modification with oxygen introduces specific reaction sites resulting in additional chemical bond formation between the vacuum‐deposited silver and the PET surface. The XPS results suggest the primary reaction sites are the plasma‐induced acid groups. The resultant structure is similar to that of a silver carboxylate species. The number of reaction sites is estimated at 3–4 times greater compared to untreated PET. These results are consistent with the significant improvement in adhesion observed for plasma treatment with oxygen.