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The functionalization of graphene (Gr) on Pt(111) and Ru(0001) substrates by an N-heterocyclic carbene (NHC) is described. The formation, thermal stability, and bonding geometry of the grafted NHC were probed using reflection absorption infrared spectroscopy (RAIRS). The carbene contains a CF3 substituent, which provides a set of three very strong absorption bands with which to monitor the chemical modification. By employing Pt(111) and Ru(0001) substrates, it is possible to compare markedly different graphene systems: while graphene forms a quasi-freestanding p-doped layer on Pt(111), its interaction with Ru(0001) involves periodic chemical bonding to form an n-doped layer. The RAIRS data show that the benzimidazolium hydrogen carbonate precursor transforms to a relatively strongly adsorbed NHC on both systems. Clean formation of the surface carbene at 300–350 K is attributed to a process activated by electron transfer from the supported graphene layer. The RAIRS signal attributed to the NHC on full-coverage Gr/Pt(111) is removed on heating to ∼450 K, while it disappears from Gr/Ru(0001) at ∼400 K. The difference in thermal stability is interpreted in terms of weaker bonding of the electron-donor NHC to the n-doped graphene layer on Ru(0001). This hypothesis is explored using oxygen intercalation to decouple graphene from Ru(0001) to form a p-doped layer. The study outlines a method to modify graphene and also reveals that vibrational spectra of an adsorbed NHC provide a highly sensitive method to probe and distinguish between different graphene-on-metal systems.